Hyperexcitability of cultured spinal motoneurons from presymptomatic ALS mice

Loss of MEF2C function by enhancer mutation leads to neuronal mitochondria dysfunction and motor deficits in mice

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by the loss of both upper and lower motor neurons, leading to progressive paralysis. Both genetic alterations and epigenetic modifications contribute to neuronal dysfunction in the pathogenesis of ALS. However, the mechanism behind genetic mutations in the non-coding region of genes that affect epigenetic modifications remains unclear.

METHODS

Convolutional neural network was used to identify an ALS-associated SNP located in the intronic region of MEF2C (rs304152), residing in a putative enhancer element. To examine the alteration of MEF2C transcription by the SNP, we generated HEK293T cells carrying the major or minor allele by CRISPR-Cas9. To verify the role of MEF2C-knockdown (MEF2C-KD) in mice, we developed AAV expressing shRNA for MEF2C based on AAV-U6 promoter vector. Neuropathological alterations of MEF2C-KD mice with mitochondrial dysfunction and motor neuronal damage were observed by confocal microscopy and transmission electron microscope (TEM). Behavioral changes of mice were examined through longitudinal study by tail suspension, inverted grid test and automated gait analysis.

RESULTS

Here, we show that enhancer mutation of MEF2C reduces own gene expression and consequently impairs mitochondrial function in motor neurons. MEF2C localizes and binds to the mitochondria DNA, and directly modulates mitochondria-encoded gene expression. CRISPR/Cas-9-induced mutation of the MEF2C enhancer decreases expression of mitochondria-encoded genes. Moreover, MEF2C mutant cells show reduction of mitochondrial membrane potential, ATP level but elevation of oxidative stress. MEF2C deficiency in the upper and lower motor neurons of mice impairs mitochondria-encoded genes, and leads to mitochondrial metabolic disruption and progressive motor behavioral deficits.

CONCLUSIONS

Together, MEF2C dysregulation by the enhancer mutation leads to mitochondrial dysfunction and oxidative stress, which are prevalent features in motor neuronal damage and ALS pathogenesis. This genetic and epigenetic crosstalk mechanism provides insights for advancing our understanding of motor neuron disease and developing effective treatments.

Spinal microcircuits go through multiphasic homeostatic compensations in a mouse model of motoneuron degeneration

In many neurological conditions, early-stage neural circuit adaptation preserves relatively normal behavior. In some diseases, spinal motoneurons progressively degenerate yet movement remains initially preserved. This study investigates whether these neurons and associated microcircuits adapt in a mouse model of progressive motoneuron degeneration. Using a combination of in vitro and in vivo electrophysiology and super-resolution microscopy, we find that, early in the disease, neurotransmission in a key pre-motor circuit, the recurrent inhibition mediated by Renshaw cells, is reduced by half due to impaired quantal size associated with decreased glycine receptor density. This impairment is specific and not a widespread feature of spinal inhibitory circuits. Furthermore, it recovers at later stages of disease. Additionally, an increased probability of release from proprioceptive afferents leads to increased monosynaptic excitation of motoneurons. We reveal that, in this motoneuron degenerative condition, spinal microcircuits undergo specific multiphasic homeostatic compensations that may contribute to preservation of force output.

Polyethyleneimine facilitates the growth and electrophysiological characterization of iPSC-derived motor neurons

Induced pluripotent stem cell (iPSC) technology, in combination with electrophysiological characterization via multielectrode array (MEA), has facilitated the utilization of iPSC-derived motor neurons (iPSC-MNs) as highly valuable models for underpinning pathogenic mechanisms and developing novel therapeutic interventions for motor neuron diseases (MNDs). However, the challenge of MN adherence to the MEA plate and the heterogeneity presented in iPSC-derived cultures raise concerns about the reproducibility of the findings obtained from these cellular models. We discovered that one novel factor modulating the electrophysiological activity of iPSC-MNs is the extracellular matrix (ECM) used in the coating to support in vitro growth, differentiation and maturation of iPSC-MNs. The current study showed that two coating conditions, namely, Poly-L-ornithine/Matrigel (POM) and Polyethyleneimine (PEI) strongly promoted attachment of iPSC-MNs on MEA culture dishes compared to three other coating conditions, and both facilitated the maturation of iPSC-MNs as characterized by the detection of extensive electrophysiological activities from the MEA plates. POM coating accelerated the maturation of the iPSC-MNs for up to 5 weeks, which suits modeling of neurodevelopmental disorders. However, the application of PEI resulted in more even distribution of the MNs on the culture dish and reduced variability of electrophysiological signals from the iPSC-MNs in 7-week cultures, which permitted the detection of enhanced excitability in iPSC-MNs from patients with amyotrophic lateral sclerosis (ALS). This study provides a comprehensive comparison of five coating conditions and offers POM and PEI as favorable coatings for in vitro modeling of neurodevelopmental and neurodegenerative disorders, respectively.

Spinal microcircuits go through multiphasic homeostatic compensations in a mouse model of motoneuron degeneration

In many neurological conditions, early-stage neural circuit adaption can preserve relatively normal behaviour. In some diseases, spinal motoneurons progressively degenerate yet movement is initially preserved. We therefore investigated whether these neurons and associated microcircuits adapt in a mouse model of progressive motoneuron degeneration. Using a combination of in vitro and in vivo electrophysiology and super-resolution microscopy, we found that, early in the disease, neurotransmission in a key pre-motor circuit, the recurrent inhibition mediated by Renshaw cells, is reduced by half due to impaired quantal size associated with decreased glycine receptor density. This impairment is specific, and not a widespread feature of spinal inhibitory circuits. Furthermore, it recovers at later stages of disease. Additionally, an increased probability of release from proprioceptive afferents leads to increased monosynaptic excitation of motoneurons. We reveal that in motoneuron degenerative conditions, spinal microcircuits undergo specific multiphasic homeostatic compensations that may contribute to preservation of force output.

ALS-like pathology diminishes swelling of spinal astrocytes in the SOD1 animal model

Astrocytes are crucial for the functioning of the nervous system as they maintain the ion homeostasis via volume regulation. Pathological states, such as amyotrophic lateral sclerosis (ALS), affect astrocytes and might even cause a loss of such functions. In this study, we examined astrocytic swelling/volume recovery in both the brain and spinal cord of the SOD1 animal model to determine the level of their impairment caused by the ALS-like pathology. Astrocyte volume changes were measured in acute brain or spinal cord slices during and after exposure to hyperkalemia. We then compared the results with alterations of extracellular space (ECS) diffusion parameters, morphological changes, expression of the Kir4.1 channel and the potassium concentration measured in the cerebrospinal fluid, to further disclose the link between potassium and astrocytes in the ALS-like pathology. Morphological analysis revealed astrogliosis in both the motor cortex and the ventral horns of the SOD1 spinal cord. The activated morphology of SOD1 spinal astrocytes was associated with the results from volume measurements, which showed decreased swelling of these cells during hyperkalemia. Furthermore, we observed lower shrinkage of ECS in the SOD1 spinal ventral horns. Immunohistochemical analysis then confirmed decreased expression of the Kir4.1 channel in the SOD1 spinal cord, which corresponded with the diminished volume regulation. Despite astrogliosis, cortical astrocytes in SOD1 mice did not show alterations in swelling nor changes in Kir4.1 expression, and we did not identify significant changes in ECS parameters. Moreover, the potassium level in the cerebrospinal fluid did not deviate from the physiological concentration. The results we obtained thus suggest that ALS-like pathology causes impaired potassium uptake associated with Kir4.1 downregulation in the spinal astrocytes, but based on our data from the cortex, the functional impairment seems to be independent of the morphological state.

Excitatory to inhibitory synaptic ratios are unchanged at presymptomatic stages in multiple models of ALS

Hyperexcitability of motor neurons and spinal cord motor circuitry has been widely reported in the early stages of Amyotrophic Lateral Sclerosis (ALS). Changes in the relative amount of excitatory to inhibitory inputs onto a neuron (E:I synaptic ratio), possibly through a developmental shift in synapse formation in favour of excitatory transmission, could underlie pathological hyperexcitability. Given that astrocytes play a major role in early synaptogenesis and are implicated in ALS pathogenesis, their potential contribution to disease mechanisms involving synaptic imbalances and subsequent hyperexcitability is also of great interest. In order to assess E:I ratios in ALS, we utilised a novel primary spinal neuron / astrocyte co-culture system, derived from neonatal mice, in which synapses are formed in vitro. Using multiple ALS mouse models we found that no combination of astrocyte or neuron genotype produced alterations in E:I synaptic ratios assessed using pre- and post-synaptic anatomical markers. Similarly, we observed that ephrin-B1, a major contact-dependent astrocytic synaptogenic protein, was not differentially expressed by ALS primary astrocytes. Further to this, analysis of E:I ratios across the entire grey matter of the lumbar spinal cord in young (post-natal day 16-19) ALS mice revealed no differences versus controls. Finally, analysis in co-cultures of human iPSC-derived motor neurons and astrocytes harbouring the pathogenic C9orf72 hexanucleotide repeat expansion showed no evidence of a bias toward excitatory versus inhibitory synapse formation. We therefore conclude, utilising multiple ALS models, that we do not observe significant changes in the relative abundance of excitatory versus inhibitory synapses as would be expected if imbalances in synaptic inputs contribute to early hyperexcitability.

Loss of MEF2C function by enhancer mutation leads to neuronal mitochondria dysfunction and motor deficits in mice

Genetic changes and epigenetic modifications are associated with neuronal dysfunction in the pathogenesis of neurodegenerative disorders. However, the mechanism behind genetic mutations in the non-coding region of genes that affect epigenetic modifications remains unclear. Here, we identified an ALS-associated SNP located in the intronic region of MEF2C (rs304152), residing in a putative enhancer element, using convolutional neural network. The enhancer mutation of MEF2C reduces own gene expression and consequently impairs mitochondrial function in motor neurons. MEF2C localizes and binds to the mitochondria DNA, and directly modulates mitochondria-encoded gene expression. CRISPR/Cas-9-induced mutation of the MEF2C enhancer decreases expression of mitochondria-encoded genes. Moreover, MEF2C mutant cells show reduction of mitochondrial membrane potential, ATP level but elevation of oxidative stress. MEF2C deficiency in the upper and lower motor neurons of mice impairs mitochondria-encoded genes, and leads to mitochondrial metabolic disruption and progressive motor behavioral deficits. Together, MEF2C dysregulation by the enhancer mutation leads to mitochondrial dysfunction and oxidative stress, which are prevalent features in motor neuronal damage and ALS pathogenesis. This genetic and epigenetic crosstalk mechanism provides insights for advancing our understanding of motor neuron disease and developing effective treatments.

Excitatory action of low frequency depolarizing GABA/glycine synaptic inputs is prevalent in prenatal spinal SOD1G93A motoneurons

Amyotrophic lateral sclerosis (ALS) is a fatal adult-onset neurodegenerative disease characterized by progressive motor neuron degeneration and muscle paralysis. Recent evidence suggests the dysfunction of inhibitory signalling in ALS motor neurons. We have shown that embryonic day (E)17.5 spinal motoneurons (MNs) of the SOD1G93A mouse model of ALS exhibit an altered chloride homeostasis. At this prenatal stage, inhibition of spinal motoneurons (MNs) is mediated by depolarizing GABAergic/glycinergic postsynaptic potentials (dGPSPs). Here, using an ex vivo preparation and patch clamp recording from MNs with a chloride equilibrium set below spike threshold, we report that low input resistance (Rin ) E17.5 MNs from the SOD1G93A ALS mouse model do not correctly integrate dGPSPs evoked by electrical stimulations of GABA/glycine inputs at different frequencies. Indeed, firing activity of most wild-type (WT) MNs with low Rin was inhibited by incoming dGPSPs, whereas low Rin SOD1G93A MNs were excited or exhibited a dual response (excited by low frequency dGPSPs and inhibited by high frequency dGPSPs). Simulation highlighted the importance of the GABA/glycine input density and showed that pure excitation could be obtained in SOD-like MNs by moving GABA/glycine input away from the cell body to dendrites. This was in agreement with confocal imaging showing a lack of peri-somatic inhibitory terminals in SOD1G93A MNs compared to WT littermates. Putative fast ALS-vulnerable MNs with low Rin are therefore lacking functional inhibition at the near-term prenatal stage. KEY POINTS: We analysed the integration of GABAergic/glycinergic synaptic events by embryonic spinal motoneurons (MNs) in a mouse model of the amyotrophic lateral sclerosis (ALS) neurodegenerative disease. We found that GABAergic/glycinergic synaptic events do not properly inhibit ALS MNs with low input resistance, most probably corresponding to future vulnerable MNs. We used a neuron model to highlight the importance of the GABA/glycine terminal location and density in the integration of the GABAergic/glycinergic synaptic events. Confocal imaging showed a lack of GABA/glycine terminals on the cell body of ALS MNs. The present study suggests that putative ALS vulnerable MNs with low Rin lack functional inhibition at the near-term stage.

Preferential potentiation of AMPA-mediated currents in brainstem hypoglossal motoneurons by subchronic exposure of mice expressing the human superoxide dismutase 1 G93A gene mutation to neurotoxicant methylmercury in vivo

The exact causes of Amyotrophic lateral sclerosis (ALS), a progressive and fatal neurological disorder due to loss of upper and/or lower motoneurons, remain elusive. Gene-environment interactions are believed to be an important factor in the development of ALS. We previously showed that in vivo exposure of mice overexpressing the human superoxide dismutase 1 (hSOD1) gene mutation (hSOD1G93A; G93A), a mouse model for ALS, to environmental neurotoxicant methylmercury (MeHg) accelerated the onset of ALS-like phenotype. Here we examined the time-course of effects of MeHg on AMPA receptor (AMPAR)-mediated currents in hypoglossal motoneurons in brainstem slices prepared from G93A, hSOD1wild-type (hWT) and non-carrier WT mice following in vivo exposure to MeHg. Mice were exposed daily to 3 ppm (approximately 0.7 mg/kg/day) MeHg via drinking water beginning at postnatal day 28 (P28) and continued until P47, 64 or 84, then acute brainstem slices were prepared, and spontaneous excitatory postsynaptic currents (sEPSCs) or AMPA-evoked currents were examined using whole cell patch-clamp recording technique. Brainstem slices of untreated littermates were prepared at the same time points to serve as control. MeHg exposure had no significant effect on either sEPSCs or AMPA-evoked currents in slices from hWT or WT mice during any of those exposure time periods under our experimental conditions. MeHg also did not cause any significant effect on sEPSCs or AMPA-currents in G93A hypoglossal motoneurons at P47 and P64. However, at P84, MeHg significantly increased amplitudes of both sEPSCs and AMPA-evoked currents in hypoglossal motineurons from G93A mice (p < 0.05), but not the sEPSC frequency, suggesting a postsynaptic action on AMPARs. MeHg exposure did not cause any significant effect on GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs). Therefore, MeHg exposure in vivo caused differential effects on AMPARs in hypoglossal motoneurons from mice with different genetic backgrounds. MeHg appears to preferentially stimulate the AMPAR-mediated currents in G93A hypoglossal motoneurons in an exposure time-dependent manner, which may contribute to the AMPAR-mediated motoneuron excitotoxicity, thereby facilitating development of ALS-like phenotype.

Pathophysiology of ion channels in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) stands as the most prevalent and severe form of motor neuron disease, affecting an estimated 2 in 100,000 individuals worldwide. It is characterized by the progressive loss of cortical, brainstem, and spinal motor neurons, ultimately resulting in muscle weakness and death. Although the etiology of ALS remains poorly understood in most cases, the remodelling of ion channels and alteration in neuronal excitability represent a hallmark of the disease, manifesting not only during the symptomatic period but also in the early pre-symptomatic stages. In this review, we delve into these alterations observed in ALS patients and preclinical disease models, and explore their consequences on neuronal activities. Furthermore, we discuss the potential of ion channels as therapeutic targets in the context of ALS.

Longitudinal changes in intrinsic motoneuron excitability in amyotrophic lateral sclerosis are dependent on disease progression

Increased amplitude of persistent inward currents (PICs) is observed in pre-symptomatic genetically modified SOD1 mice models of amyotrophic lateral sclerosis (ALS). However, at the symptomatic stage this reverses and there is a large reduction in PIC amplitude. It remains unclear whether these changes in PICs can be observed in humans, with cross-sectional studies in humans reporting contradictory findings. In people with ALS, we estimated the PIC contribution to self-sustained firing of motoneurons, using the paired-motor unit analysis to calculate the Δfrequency (ΔF), to compare the weaker and stronger muscles during the course of disease. We hypothesised that, with disease progression, ΔFs would relatively increase in the stronger muscles; and decline in the weaker muscles. Forty-three individuals with ALS were assessed in two occasions on average 17 weeks apart. Tibialis anterior high-density electromyograms were recorded during dorsiflexion (40% of maximal capacity) ramped contractions, followed by clinical tests. ∆F increased from 3.14 (2.57, 3.71) peaks per second (pps) to 3.55 (2.94, 4.17) pps on the stronger muscles (0.41 (0.041, 0.781) pps, standardised difference (d) = 0.287 (0.023, 0.552), P = 0.030). ∆F reduced from 3.38 (95% CI 2.92, 3.84) pps to 2.88 (2.40, 3.36) pps on the weaker muscles (-0.50 (-0.80, -0.21) pps, d = 0.353 (0.138, 0.567), P = 0.001). The ALSFRS-R score reduced 3.9 (2.3, 5.5) points. These data indicate that the contribution of PICs to motoneuron self-sustained firing increases over time in early stages of the disease when there is little weakness before decreasing as the disease progresses and muscle weakness exacerbates, in alignment with the findings from studies using SOD1 mice. KEY POINTS: Research on mouse model of amyotrophic lateral sclerosis (ALS) suggests that the amplitude of persistent inward currents (PICs) is increased in early stages before decreasing as the disease progresses. Cross-sectional studies in humans have reported contradictory findings with both higher and lower PIC contributions to motoneuron self-sustained firing. In this longitudinal (∼17 weeks) study we tracked changes in PIC contribution to motoneuron self-sustained firing, using the ΔF calculation (i.e. onset-offset hysteresis of motor unit pairs), in tibialis anterior muscles with normal strength and with clinical signs of weakness in people with ALS. ΔFs decreased over time in muscles with clinical signs of weakness. The PIC contribution to motoneuron self-sustained firing increases before the onset of muscle weakness, and subsequently decreases when muscle weakness progresses.

A novel protocol to derive cervical motor neurons from induced pluripotent stem cells for amyotrophic lateral sclerosis

Sporadic amyotrophic lateral sclerosis (sALS) is the majority of ALS, and the lack of appropriate disease models has hindered its research. Induced pluripotent stem cell (iPSC) technology now permits derivation of iPSCs from somatic cells of sALS patients to investigate disease phenotypes and mechanisms. Most existing differentiation protocols are time-consuming or low efficient in generating motor neurons (MNs). Here we report a rapid and simple protocol to differentiate MNs in monolayer culture using small molecules, which led to nearly pure neural stem cells in 6 days, robust OLIG2+ pMNs (73%-91%) in 12 days, enriched CHAT+ cervical spinal MNs (sMNs) (88%-97%) in 18 days, and functionally mature sMNs in 28 days. This simple and reproducible protocol permitted the identification of hyperexcitability phenotypes in our sALS iPSC-derived sMNs, and its application in neurodegenerative diseases should facilitate in vitro disease modeling, drug screening, and the development of cell therapy.

An impaired splicing program underlies differentiation defects in hSOD1G93A neural progenitor cells

Amyotrophic lateral sclerosis (ALS) is an adult devastating neurodegenerative disease characterized by the loss of upper and lower motor neurons (MNs), resulting in progressive paralysis and death. Genetic animal models of ALS have highlighted dysregulation of synaptic structure and function as a pathogenic feature of ALS-onset and progression. Alternative pre-mRNA splicing (AS), which allows expansion of the coding power of genomes by generating multiple transcript isoforms from each gene, is widely associated with synapse formation and functional specification. Deciphering the link between aberrant splicing regulation and pathogenic features of ALS could pave the ground for novel therapeutic opportunities. Herein, we found that neural progenitor cells (NPCs) derived from the hSOD1G93A mouse model of ALS displayed increased proliferation and propensity to differentiate into neurons. In parallel, hSOD1G93A NPCs showed impaired splicing patterns in synaptic genes, which could contribute to the observed phenotype. Remarkably, master splicing regulators of the switch from stemness to neural differentiation are de-regulated in hSOD1G93A NPCs, thus impacting the differentiation program. Our data indicate that hSOD1G93A mutation impacts on neurogenesis by increasing the NPC pool in the developing mouse cortex and affecting their intrinsic properties, through the establishment of a specific splicing program.

Early deficits in GABA inhibition parallels an increase in L-type Ca2+ currents in the jaw motor neurons of SOD1G93A mouse model for ALS

Amyotrophic Lateral Sclerosis (ALS) involves protracted pre-symptomatic periods of abnormal motor neuron (MN) excitability occurring in parallel with central and peripheral synaptic perturbations. Focusing on inhibitory control of MNs, we first compared longitudinal changes in pre-synaptic terminal proteins for GABA and glycine neurotransmitters around the soma of retrogradely identified trigeminal jaw closer (JC) MNs and ChAT-labeled midbrain extraocular (EO) MNs in the SOD1^G93A mouse model for ALS. Fluorescence immunocytochemistry and confocal imaging were used to quantify GAD67 and GlyT2 synaptic bouton density (SBD) around MN soma at pre-symptomatic ages ∼P12 (postnatal), ∼P50 (adult) and near disease end-stage (∼P135) in SOD1^G93A mice and age-matched wild-type (WT) controls. We noted reduced GAD67 innervation in the SOD1^G93A trigeminal jaw closer MNs around P12, relative to age-matched WT and no significant difference around P50 and P135. In contrast, both GAD67 and GlyT2 innervation were elevated in the SOD1^G93A EO MNs at the pre-symptomatic time points. Considering trigeminal MNs are vulnerable in ALS while EO MNs are spared, we suggest that upregulation of inhibition in the latter might be compensatory. Notable contrast also existed in the innate co-expression patterns of GAD67 and GlyT2 with higher mutual information (co-dependency) in EO MNs compared to JC in both SOD1^G93A and WT mice, especially at adult stages (P50 and P135). Around P12 when GAD67 terminals expression was low in the mutant, we further tested for persistent GABA inhibition in those MNs using in vitro patch-clamp electrophysiology. Our results show that SOD1^G93A JC MNs have reduced persistent GABA inhibition, relative to WT. Pharmacological blocking of an underlying tonically active GABA conductance using the GABA-α5 subunit inverse agonist, L-655-708, disinhibited WT JC MNs and lowered their recruitment threshold, suggesting its role in the control of intrinsic MN excitability. Quantitative RT-PCR in laser dissected JC MNs further supported a reduction in GABA-α5 subunit mRNA expression in the mutant. In light of our previous report that JC MNs forming putative fast motor units have lower input threshold in the SOD1^G93A mice, we suggest that our present result on reduced GABA-α5 tonic inhibition provides for a mechanism contributing to such imbalance. In parallel with reduced GABA inhibition, we noted an increase in voltage-gated L-type Ca²⁺ currents in the mutant JC MNs around P12. Together these results support that, early modifications in intrinsic properties of vulnerable MNs could be an adaptive response to counter synaptic deficits.

Inhibitory interneurons show early dysfunction in a SOD1 mouse model of amyotrophic lateral sclerosis

Few studies in amyotrophic lateral sclerosis (ALS) measure effects of the disease on inhibitory interneurons synapsing onto motoneurons (MNs). However, inhibitory interneurons could contribute to dysfunction, particularly if altered before MN neuropathology, and establish a long-term imbalance of inhibition/excitation. We directly assessed excitability and morphology of glycinergic (GlyT2 expressing) ventral lumbar interneurons from SOD1G93AGlyT2eGFP (SOD1) and wild-type GlyT2eGFP (WT) mice on postnatal days 6-10. Patch clamp revealed dampened excitability in SOD1 interneurons, including depolarized persistent inward currents (PICs), increased voltage and current threshold for firing action potentials, along with a marginal decrease in afterhyperpolarization duration. Primary neurites of ventral SOD1 inhibitory interneurons were larger in volume and surface area than WT. GlyT2 interneurons were then divided into three subgroups based on location: (1) interneurons within 100 μm of the ventral white matter, where Renshaw cells (RCs) are located, (2) interneurons interspersed with MNs in lamina IX, and (3) interneurons in the intermediate ventral area including laminae VII and VIII. Ventral interneurons in the RC area were the most profoundly affected, exhibiting more depolarized PICs and larger primary neurites. Interneurons in lamina IX had depolarized PIC onset. In lamina VII-VIII, interneurons were least affected. In summary, inhibitory interneurons show very early region-specific perturbations poised to impact excitatory/inhibitory balance of MNs, modify motor output and provide early biomarkers of ALS. Therapeutics like riluzole that universally reduce CNS excitability could exacerbate the inhibitory dysfunction described here. KEY POINTS: Spinal inhibitory interneurons could contribute to amyotrophic lateral sclerosis (ALS) pathology, but their excitability has never been directly measured. We studied the excitability and morphology of glycinergic interneurons in early postnatal transgenic mice (SOD1G93A GlyT2eGFP). Interneurons were less excitable and had marginally smaller somas but larger primary neurites in SOD1 mice. GlyT2 interneurons were analysed according to their localization within the ventral spinal cord. Interestingly, the greatest differences were observed in the most ventrally located interneurons. We conclude that inhibitory interneurons show presymptomatic changes that may contribute to excitatory/inhibitory imbalance in ALS.

Motoneuron excitability dysfunction in ALS: Pseudo-mystery or authentic conundrum?

In amyotrophic lateral sclerosis (ALS), abnormalities in motoneuronal excitability are seen in early pathogenesis and throughout disease progression. Fully understanding motoneuron excitability dysfunction may lead to more effective treatments. Yet decades of research have not produced consensus on the nature, role or underlying mechanisms of motoneuron excitability dysfunction in ALS. For example, contrary to Ca excitotoxicity theory, predictions of motoneuronal hyper-excitability, normal and hypo-excitability have also been seen at various disease stages and in multiple ALS lines. Accordingly, motoneuron excitability dysfunction in ALS is a disputed topic in the field. Specifically, the form (hyper, hypo or unchanged) and what role excitability dysfunction plays in the disease (pathogenic or downstream of other pathologies; neuroprotective or detrimental) are currently unclear. Although several motoneuron properties that determine cellular excitability change in the disease, some of these changes are pro-excitable, whereas others are anti-excitable, making dynamic fluctuations in overall 'net' excitability highly probable. Because various studies assess excitability via differing methods and at differing disease stages, the conflicting reports in the literature are not surprising. Hence, the overarching process of excitability degradation and motoneuron degeneration is not fully understood. Consequently, the discrepancies on motoneuron excitability dysfunction in the literature represent a substantial barrier to our understanding of the disease. Emerging studies suggest that biological variables, variations in experimental protocols, issues of rigor and sampling/analysis strategies are key factors that may underlie conflicting data in the literature. This review highlights potential confounding factors for researchers to consider and also offers ideas on avoiding pitfalls and improving robustness of data.

Non-Invasive Transcutaneous Spinal DC Stimulation as a Neurorehabilitation ALS Therapy in Awake G93A Mice: The First Step to Clinical Translation

Spinal direct current stimulation (sDCS) modulates motoneuron (MN) excitability beyond the stimulation period, making it a potential neurorehabilitation therapy for amyotrophic lateral sclerosis (ALS), a MN degenerative disease in which MN excitability dysfunction plays a critical and complex role. Recent evidence confirms induced changes in MN excitability via measured MN electrophysiological properties in the SOD1 ALS mouse during and following invasive subcutaneous sDCS (ssDCS). The first aim of our pilot study was to determine the clinical potential of these excitability changes at symptom onset (P90-P105) in ALS via a novel non-invasive transcutaneous sDCS (tsDCS) treatment paradigm on un-anesthetized SOD1-G93A mice. The primary outcomes were motor function and survival. Unfortunately, skin damage avoidance limited the strength of applied stimulation intensity, likewise limiting measurable primary effects. The second aim of this study was to determine which orientation of stimulation (anodal vs cathodal, which are expected to have opposing effects) is beneficial vs harmful in ALS. Despite the lack of measured primary effects, strong trends in survival of the anodal stimulation group, combined with an analysis of survival variance and correlations among symptoms, suggest anodal stimulation is harmful at symptom onset. Therefore, cathodal stimulation may be beneficial at symptom onset if a higher stimulation intensity can be safely achieved via subcutaneously implanted electrodes or alternative methods. Importantly, the many logistical, physical, and stimulation parameters explored in developing this novel non-invasive treatment paradigm on unanesthetized mice provide insight into an appropriate and feasible methodology for future tsDCS study designs and potential clinical translation.

Array tomography: 15 years of synaptic analysis

Synapses are minuscule, intricate structures crucial for the correct communication between neurons. In the 125 years since the term synapse was first coined, we have advanced a long way when it comes to our understanding of how they work and what they do. Most of the fundamental discoveries have been invariably linked to advances in technology. However, due to their size, delicate structural integrity and their sheer number, our knowledge of synaptic biology has remained somewhat elusive and their role in neurodegenerative diseases still remains largely unknown. Here, we briefly discuss some of the imaging technologies used to study synapses and focus on the utility of the high-resolution imaging technique array tomography (AT). We introduce the AT technique and highlight some of the ways it is utilised with a particular focus on its power for analysing synaptic composition and pathology in human post-mortem tissue. We also discuss some of the benefits and drawbacks of techniques for imaging synapses and highlight some recent advances in the study of form and function by combining physiology and high-resolution synaptic imaging.

Detecting motor unit abnormalities in amyotrophic lateral sclerosis using high-density surface EMG

OBJECTIVE

The purpose of this study was to detect specific motor unit (MU) abnormalities in people with amyotrophic lateral sclerosis (ALS) compared to controls using high-density surface electromyography (HD-SEMG).

METHODS

Sixteen people with ALS and 16 control subjects. The participants performed ramp up and sustained contractions at 30% of their maximal voluntary contraction. HD-SEMG signals were recorded in the vastus lateralis muscle and decomposed into individual MU firing behavior using a convolution blind source separation method.

RESULTS

In total, 339 MUs were detected (people with ALS; n = 93, control subjects; n = 246). People with ALS showed significantly higher mean firing rate, recruitment threshold, coefficient of variation of the MU firing rate, MU firing rate at recruitment, and motoneurons excitability than those of control subjects (p < 0.001). The number of MU, MU firing rate, recruitment threshold, and MU firing rate at recruitment were significantly correlated with disease severity (p < 0.001). Multivariable analysis revealed that an increased MU firing rate at recruitment was independently associated with ALS.

CONCLUSIONS

These results suggest increased excitability at recruitment, which is consistent with neurodegeneration results in a compensatory increase in MU activity.

SIGNIFICANCE

Abnormal MU firing behavior provides an important physiological index for understanding the pathophysiology of ALS.

Aging-Dependent Altered Transcriptional Programs Underlie Activity Impairments in Human C9orf72-Mutant Motor Neurons

Amyotrophic Lateral Sclerosis (ALS) is an incurable neurodegenerative disease characterized by dysfunction and loss of upper and lower motor neurons (MN). Despite several studies identifying drastic alterations affecting synaptic composition and functionality in different experimental models, the specific contribution of impaired activity to the neurodegenerative processes observed in ALS-related MN remains controversial. In particular, contrasting lines of evidence have shown both hyper- as well as hypoexcitability as driving pathomechanisms characterizing this specific neuronal population. In this study, we combined high definition multielectrode array (HD-MEA) techniques with transcriptomic analysis to longitudinally monitor and untangle the activity-dependent alterations arising in human C9orf72-mutant MN. We found a time-dependent reduction of neuronal activity in ALSC9orf72 cultures occurring as synaptic contacts undergo maturation and matched by a significant loss of mutant MN upon aging. Notably, ALS-related neurons displayed reduced network synchronicity most pronounced at later stages of culture, suggesting synaptic imbalance. In concordance with the HD-MEA data, transcriptomic analysis revealed an early up-regulation of synaptic terms in ALSC9orf72 MN, whose expression was decreased in aged cultures. In addition, treatment of older mutant cells with Apamin, a K+ channel blocker previously shown to be neuroprotective in ALS, rescued the time-dependent loss of firing properties observed in ALSC9orf72 MN as well as the expression of maturity-related synaptic genes. All in all, this study broadens the understanding of how impaired synaptic activity contributes to MN degeneration in ALS by correlating electrophysiological alterations to aging-dependent transcriptional programs.

Neuromuscular Junction Dysfunction in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disorder characterized by progressive degeneration of motor neurons leading to skeletal muscle denervation. Earlier studies have shown that motor neuron degeneration begins in motor cortex and descends to the neuromuscular junction (NMJ) in a dying forward fashion. However, accumulating evidences support that ALS is a distal axonopathy where early pathological changes occur at the NMJ, prior to onset of clinical symptoms and propagates towards the motor neuron cell body supporting "dying back" hypothesis. Despite several evidences, series of events triggering NMJ disassembly in ALS are still obscure. Neuromuscular junction is a specialized tripartite chemical synapse which involves a well-coordinated communication among the presynaptic motor neuron, postsynaptic skeletal muscle, and terminal Schwann cells. This review provides comprehensive insight into the role of NMJ in ALS pathogenesis. We have emphasized the molecular alterations in cellular components of NMJ leading to loss of effective neuromuscular transmission in ALS. Further, we provide a preview into research involved in exploring NMJ as potential target for designing effective therapies for ALS.

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

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

OUP accepted manuscript

Amyotrophic lateral sclerosis is a late-onset adult neurodegenerative disease, although there is mounting electrophysiological and pathological evidence from patients and animal models for a protracted preclinical period of motor neuron susceptibility and dysfunction, long before clinical diagnosis. The key molecular mechanisms linked to motor neuron vulnerability in amyotrophic lateral sclerosis have been extensively studied using transcriptional profiling in motor neurons isolated from adult mutant superoxide dismutase 1 mice. However, neonatal and embryonic motor neurons from mutant superoxide dismutase 1 mice show abnormal morphology and hyperexcitability, suggesting preceding transcriptional dysregulation. Here, we used RNA sequencing on motor neurons isolated from embryonic superoxide dismutase 1^G93A mice to determine the earliest molecular mechanisms conferring neuronal susceptibility and dysfunction known in a mouse model of amyotrophic lateral sclerosis. Transgenic superoxide dismutase 1^G93A mice expressing the spinal motor neuron homeobox HB9:green fluorescent protein reporter allowed unambiguous identification and isolation of motor neurons using fluorescence-activated cell sorting. Gene expression profiling of isolated motor neurons revealed transcriptional dysregulation in superoxide dismutase 1^G93A mice as early as embryonic Day 12.5 with the majority of differentially expressed genes involved in RNA processing and α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate-mediated glutamate receptor signalling. We confirmed dysregulation of the α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor Subunit 2, at transcript and protein levels, in embryonic superoxide dismutase 1^G93A mouse motor neurons and human motor neurons derived from amyotrophic lateral sclerosis patient induced pluripotent stem cells harbouring pathogenic superoxide dismutase 1 mutations. Mutant superoxide dismutase 1 induced pluripotent stem cell-derived motor neurons showed greater vulnerability to α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate-mediated excitotoxicity, consistent with α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor Subunit 2 downregulation. Thus, α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor Subunit 2 deficiency leading to enhanced α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor signalling, calcium influx, hyperexcitability, and chronic excitotoxicity is a very early and intrinsic property of spinal motor neurons that may trigger amyotrophic lateral sclerosis pathogenesis later in life. This study reinforces the concept of therapeutic targeting of hyperexcitability and excitotoxicity as potential disease-modifying approaches for amyotrophic lateral sclerosis.

From Physiological Properties to Selective Vulnerability of Motor Units in Amyotrophic Lateral Sclerosis

Spinal alpha-motoneurons are classified in several types depending on the contractile properties of the innervated muscle fibers. This diversity is further displayed in different levels of vulnerability of distinct motor units to neurodegenerative diseases such as Amyotrophic Lateral Sclerosis (ALS). We summarize recent data suggesting that, contrary to the excitotoxicity hypothesis, the most vulnerable motor units are hypoexcitable and experience a reduction in their firing prior to symptoms onset in ALS. We suggest that a dysregulation of activity-dependent transcriptional programs in these motoneurons alter crucial cellular functions such as mitochondrial biogenesis, autophagy, axonal sprouting capability and re-innervation of neuromuscular junctions.

Slo2/KNa Channels in Drosophila Protect against Spontaneous and Induced Seizure-like Behavior Associated with an Increased Persistent Na+ Current

Na⁺ sensitivity is a unique feature of Na⁺-activated K⁺ (K_Na) channels, making them naturally suited to counter a sudden influx in Na⁺ ions. As such, it has long been suggested that K_Na channels may serve a protective function against excessive excitation associated with neuronal injury and disease. This hypothesis, however, has remained largely untested. Here, we examine K_Na channels encoded by the Drosophila Slo2 (dSlo2) gene in males and females. We show that dSlo2/K_Na channels are selectively expressed in cholinergic neurons in the adult brain, as well as in glutamatergic motor neurons, where dampening excitation may function to inhibit global hyperactivity and seizure-like behavior. Indeed, we show that effects of feeding Drosophila a cholinergic agonist are exacerbated by the loss of dSlo2/K_Na channels. Similar to mammalian Slo2/K_Na channels, we show that dSlo2/K_Na channels encode a TTX-sensitive K⁺ conductance, indicating that dSlo2/K_Na channels can be activated by Na⁺ carried by voltage-dependent Na⁺ channels. We then tested the role of dSlo2/K_Na channels in established genetic seizure models in which the voltage-dependent persistent Na⁺ current (I_Nap) is elevated. We show that the absence of dSlo2/K_Na channels increased susceptibility to mechanically induced seizure-like behavior. Similar results were observed in WT flies treated with veratridine, an enhancer of I_Nap Finally, we show that loss of dSlo2/K_Na channels in both genetic and pharmacologically primed seizure models resulted in the appearance of spontaneous seizures. Together, our results support a model in which dSlo2/K_Na channels, activated by neuronal overexcitation, contribute to a protective threshold to suppress the induction of seizure-like activity.SIGNIFICANCE STATEMENT Slo2/K_Na channels are unique in that they constitute a repolarizing K⁺ pore that is activated by the depolarizing Na⁺ ion, making them naturally suited to function as a protective "brake" against overexcitation and Na⁺ overload. Here, we test this hypothesis in vivo by examining how a null mutation of the Drosophila Slo2 (dSlo2)/K_Na gene affects seizure-like behavior in genetic and pharmacological models of epilepsy. We show that indeed the loss of dSlo2/K_Na channels results in increased incidence and severity of induced seizure behavior, as well as the appearance of spontaneous seizure activity. Our results advance our understanding of neuronal excitability and protective mechanisms that preserve normal physiology and the suppression of seizure susceptibility.

Neurophysiological Mechanisms Underlying Cortical Hyper-Excitability in Amyotrophic Lateral Sclerosis: A Review

Amyotrophic lateral sclerosis (ALS) is a progressive neuromotor disease characterized by the loss of upper and lower motor neurons (MNs), resulting in muscle paralysis and death. Early cortical hyper-excitability is a common pathological process observed clinically and in animal disease models. Although the mechanisms that underlie cortical hyper-excitability are not completely understood, the molecular and cellular mechanisms that cause enhanced neuronal intrinsic excitability and changes in excitatory and inhibitory synaptic activity are starting to emerge. Here, we review the evidence for an anterograde glutamatergic excitotoxic process, leading to cortical hyper-excitability via intrinsic cellular and synaptic mechanisms and for the role of interneurons in establishing disinhibition in clinical and experimental settings. Understanding the mechanisms that lead to these complex pathological processes will likely produce key insights towards developing novel therapeutic strategies to rescue upper MNs, thus alleviating the impact of this fatal disease.

Time Course of Alterations in Adult Spinal Motoneuron Properties in the SOD1(G93A) Mouse Model of ALS

Although amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease, motoneuron electrical properties are already altered during embryonic development. Motoneurons must therefore exhibit a remarkable capacity for homeostatic regulation to maintain a normal motor output for most of the life of the patient. In the present article, we demonstrate how maintaining homeostasis could come at a very high cost. We studied the excitability of spinal motoneurons from young adult SOD1(G93A) mice to end-stage. Initially, homeostasis is highly successful in maintaining their overall excitability. This initial success, however, is achieved by pushing some cells far above the normal range of passive and active conductances. As the disease progresses, both passive and active conductances shrink below normal values in the surviving cells. This shrinkage may thus promote survival, implying the previously large values contribute to degeneration. These results support the hypothesis that motoneuronal homeostasis may be "hypervigilant" in ALS and a source of accumulating stress.

Diaphragm neuromuscular transmission failure in a mouse model of an early-onset neuromotor disorder

The spa transgenic mouse displays spasticity and hypertonia that develops during the early postnatal period, with motor impairments that are remarkably similar to symptoms of human cerebral palsy. Previously, we observed that spa mice have fewer phrenic motor neurons innervating the diaphragm muscle (DIAm). We hypothesize that spa mice exhibit increased susceptibility to neuromuscular transmission failure (NMTF) due to an expanded innervation ratio. We retrogradely labeled phrenic motor neurons with rhodamine and imaged them in horizontal sections (70 µm) using confocal microscopy. Phrenic nerve-DIAm strip preparations from wild type and spa mice were stretched to optimal length, and force was evoked by phrenic nerve stimulation at 10, 40, or 75 Hz in 330-ms duration trains repeated each second (33% duty cycle) across a 120-s period. To assess NMTF, force evoked by phrenic nerve stimulation was compared to force evoked by direct DIAm stimulation superimposed every 15 s. Total DIAm fiber number was estimated in hematoxylin and eosin-stained strips. Compared to wild type, spa mice had over twofold greater NMTF during the first stimulus train that persisted throughout the 120 s period of repetitive activation. In both wild type and spa mice, NMTF was stimulation-frequency dependent. There was no difference in neuromuscular junction morphology or the total number of DIAm fibers between wild type and spa mice, however, there was an increase innervation ratio (39%) in spa mice. We conclude that early-onset developmental neuromotor disorders impair the efficacy of DIAm neuromuscular transmission, likely to contribute to respiratory complications.NEW & NOTEWORTHY Individuals with motor control deficits, including cerebral palsy (CP) often have respiratory impairments. Glycine-receptor mutant spa mice have early-onset hypertonia, and limb motor impairments, similar to individuals with CP. We hypothesized that in the diaphragm of spa mice, disruption of glycinergic inputs to MNs would result in increased phrenic-DIAm neuromuscular transmission failure. Pathophysiologic abnormalities in neuromuscular transmission may contribute to respiratory dysfunction in conditions where early developmental MN loss or motor control deficits are apparent.

How Degeneration of Cells Surrounding Motoneurons Contributes to Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurological disorder characterized by the progressive degeneration of upper and lower motoneurons. Despite motoneuron death being recognized as the cardinal event of the disease, the loss of glial cells and interneurons in the brain and spinal cord accompanies and even precedes motoneuron elimination. In this review, we provide striking evidence that the degeneration of astrocytes and oligodendrocytes, in addition to inhibitory and modulatory interneurons, disrupt the functionally coherent environment of motoneurons. We discuss the extent to which the degeneration of glial cells and interneurons also contributes to the decline of the motor system. This pathogenic cellular network therefore represents a novel strategic field of therapeutic investigation.

Increased Axon Initial Segment Length Results in Increased Na+ Currents in Spinal Motoneurones at Symptom Onset in the G127X SOD1 Mouse Model of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease preferentially affecting motoneurones. Transgenic mouse models have been used to investigate the role of abnormal motoneurone excitability in this disease. Whilst an increased excitability has repeatedly been demonstrated in vitro in neonatal and embryonic preparations from SOD1 mouse models, the results from the only studies to record in vivo from spinal motoneurones in adult SOD1 models have produced conflicting findings. Deficits in repetitive firing have been reported in G93A SOD1^{(high copy number)} mice but not in presymptomatic G127X SOD1 mice despite shorter motoneurone axon initial segments (AISs) in these mice. These discrepancies may be due to the earlier disease onset and prolonged disease progression in G93A SOD1 mice with recordings potentially performed at a later sub-clinical stage of the disease in this mouse. To test this, and to explore how the evolution of excitability changes with symptom onset we performed in vivo intracellular recording and AIS labelling in G127X SOD1 mice immediately after symptom onset. No reductions in repetitive firing were observed showing that this is not a common feature across all ALS models. Immunohistochemistry for the Na⁺ channel Nav1.6 showed that motoneurone AISs increase in length in G127X SOD1 mice at symptom onset. Consistent with this, the rate of rise of AIS components of antidromic action potentials were significantly faster confirming that this increase in length represents an increase in AIS Na⁺ channels occurring at symptom onset in this model.

Spinal motoneurones are intrinsically more responsive in the adult G93A SOD1 mouse model of amyotrophic lateral sclerosis

KEY POINTS

Although in vitro recordings using neonatal preparations from mouse models of amyotrophic lateral sclerosis (ALS) suggest increased motoneurone excitability, in vivo recordings in adult ALS mouse models have been conflicting. In adult G93A SOD1 models, spinal motoneurones have previously been shown to have deficits in repetitive firing, in contrast to the G127X SOD1 mouse model. Our in vivo intracellular recordings in barbiturate-anaesthetized adult male G93A SOD1 mice reveal that the incidence of failure to fire with current injection was equally low in control and ALS mice (∼2%). We show that failure to fire repetitively can be a consequence of experimental protocol and should not be used alone to classify otherwise normal motoneurones as hypo-excitable. Motoneurones in the G93A SOD1 mice showed an increased response to inputs, with lower rheobase, higher input-output gains and increased activation of persistent inward currents.

ABSTRACT

In vitro studies from transgenic amyotrophic lateral sclerosis models have suggested an increased excitability of spinal motoneurones. However, in vivo intracellular recordings from adult amyotrophic lateral sclerosis mice models have produced conflicting findings. Previous investigations using barbiturate anaesthetized G93A SOD1 mice have suggested that some motoneurones are hypo-excitable, defined by deficits in repetitive firing. Our own previous recordings in G127X SOD1 mice using different anaesthesia, however, showed no repetitive firing deficits and increased persistent inward currents at symptom onset. These discrepancies may be a result of differences between models, symptomatic stage, anaesthesia or technical differences. To investigate this, we repeated our original experiments, but in adult male G93A SOD1 mice, at both presymptomatic and symptomatic stages, under barbiturate anaesthesia. In vivo intracellular recordings from antidromically identified spinal motoneurones revealed that the incidence of failure to fire with current injection was equally low in control and G93A SOD1 mice (∼2%). Motoneurones in G93A SOD1 mice fired significantly more spontaneous action potentials. Rheobase was significantly lower and the input resistance and input-output gain were significantly higher in both presymptomatic and symptomatic G93A SOD1 mice. This was despite a significant increase in the duration of the post-spike after-hyperpolarization in both presymptomatic and symptomatic G93A SOD1 mice. Finally, evidence of increased activation of persistent inward currents was seen in both presymptomatic and symptomatic G93A SOD1 mice. Our results do not confirm previous reports of hypo-excitability of spinal motoneurones in the G93A SOD1 mouse and demonstrate that the motoneurones show an increased response to inputs.

Exciting Complexity: The Role of Motor Circuit Elements in ALS Pathophysiology

Amyotrophic lateral sclerosis (ALS) is a fatal disease, characterized by the degeneration of both upper and lower motor neurons. Despite decades of research, we still to date lack a cure or disease modifying treatment, emphasizing the need for a much-improved insight into disease mechanisms and cell type vulnerability. Altered neuronal excitability is a common phenomenon reported in ALS patients, as well as in animal models of the disease, but the cellular and circuit processes involved, as well as the causal relevance of those observations to molecular alterations and final cell death, remain poorly understood. Here, we review evidence from clinical studies, cell type-specific electrophysiology, genetic manipulations and molecular characterizations in animal models and culture experiments, which argue for a causal involvement of complex alterations of structure, function and connectivity of different neuronal subtypes within the cortical and spinal cord motor circuitries. We also summarize the current knowledge regarding the detrimental role of astrocytes and reassess the frequently proposed hypothesis of glutamate-mediated excitotoxicity with respect to changes in neuronal excitability. Together, these findings suggest multifaceted cell type-, brain area- and disease stage- specific disturbances of the excitation/inhibition balance as a cardinal aspect of ALS pathophysiology.

Excessive Homeostatic Gain in Spinal Motoneurons in a Mouse Model of Amyotrophic Lateral Sclerosis

In the mSOD1 model of ALS, the excitability of motoneurons is poorly controlled, oscillating between hyperexcitable and hypoexcitable states during disease progression. The hyperexcitability is mediated by excessive activity of voltage-gated Na⁺ and Ca²⁺ channels that is initially counteracted by aberrant increases in cell size and conductance. The balance between these opposing actions collapses, however, at the time that the denervation of muscle fibers begins at about P50, resulting in a state of hypo-excitability and cell death. We propose that this process of neurodegeneration ensues from homeostatic dysregulation of excitability and have tested this hypothesis by perturbing a signal transduction pathway that plays a major role in controlling biogenesis and cell size. Our 『homeostatic dysregulation hypothesis' predicted that neonatal mSOD1 motoneurons would be much more sensitive to such perturbations than wild type controls and our results strongly support this hypothesis. Our results have important implications for therapeutic approaches to ALS.

Motoneuronal Spinal Circuits in Degenerative Motoneuron Disease

The most evident phenotype of degenerative motoneuron disease is the loss of motor function which accompanies motoneuron death. In both amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA), it is now clear that dysfunction is not restricted to motoneurons but is manifest in the spinal circuits in which motoneurons are embedded. As mounting evidence shows that motoneurons possess more elaborate and extensive connections within the spinal cord than previously realized, it is necessary to consider the role of this circuitry and its dysfunction in the disease process. In this review article, we ask if the selective vulnerability of the different motoneuron types and the relative disease resistance of distinct motoneuron groups can be understood in terms of their intraspinal connections.

Abnormal Upregulation of GPR17 Receptor Contributes to Oligodendrocyte Dysfunction in SOD1 G93A Mice

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive loss of motor neurons (MN). Importantly, MN degeneration is intimately linked to oligodendrocyte dysfunction and impaired capacity of oligodendrocyte precursor cells (OPCs) to regenerate the myelin sheath enwrapping and protecting neuronal axons. Thus, improving OPC reparative abilities represents an innovative approach to counteract MN loss. A pivotal regulator of OPC maturation is the P2Y-like G protein-coupled receptor 17 (GPR17), whose role in ALS has never been investigated. In other models of neurodegeneration, an abnormal increase of GPR17 has been invariably associated to myelin defects and its pharmacological manipulation succeeded in restoring endogenous remyelination. Here, we analyzed GPR17 alterations in the SOD1^G93A ALS mouse model and assessed in vitro whether this receptor could be targeted to correct oligodendrocyte alterations. Western-blot and immunohistochemical analyses showed that GPR17 protein levels are significantly increased in spinal cord of ALS mice at pre-symptomatic stage; this alteration is exacerbated at late symptomatic phases. Concomitantly, mature oligodendrocytes degenerate and are not successfully replaced. Moreover, OPCs isolated from spinal cord of SOD1^G93A mice display defective differentiation compared to control cells, which is rescued by treatment with the GPR17 antagonist montelukast. These data open novel therapeutic perspectives for ALS management.

Shortened TDP43 isoforms upregulated by neuronal hyperactivity drive TDP43 pathology in ALS

Cortical hyperexcitability and mislocalization of the RNA-binding protein TDP43 are highly conserved features in amyotrophic lateral sclerosis (ALS). Nevertheless, the relationship between these phenomena remains poorly defined. Here, we showed that hyperexcitability recapitulates TDP43 pathology by upregulating shortened TDP43 (sTDP43) splice isoforms. These truncated isoforms accumulated in the cytoplasm and formed insoluble inclusions that sequestered full-length TDP43 via preserved N-terminal interactions. Consistent with these findings, sTDP43 overexpression was toxic to mammalian neurons, suggesting neurodegeneration arising from complementary gain- and loss-of-function mechanisms. In humans and mice, sTDP43 transcripts were enriched in vulnerable motor neurons, and we observed a striking accumulation of sTDP43 within neurons and glia of ALS patients. Collectively, these studies uncover a pathogenic role for alternative TDP43 isoforms in ALS, and implicate sTDP43 as a key contributor to the susceptibility of motor neurons in this disorder.

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

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

Mutant C9orf72 human iPSC-derived astrocytes cause non-cell autonomous motor neuron pathophysiology

Mutations in C9orf72 are the most common genetic cause of amyotrophic lateral sclerosis (ALS). Accumulating evidence implicates astrocytes as important non‐cell autonomous contributors to ALS pathogenesis, although the potential deleterious effects of astrocytes on the function of motor neurons remains to be determined in a completely humanized model of C9orf72‐mediated ALS. Here, we use a human iPSC‐based model to study the cell autonomous and non‐autonomous consequences of mutant C9orf72 expression by astrocytes. We show that mutant astrocytes both recapitulate key aspects of C9orf72‐related ALS pathology and, upon co‐culture, cause motor neurons to undergo a progressive loss of action potential output due to decreases in the magnitude of voltage‐activated Na⁺ and K⁺ currents. Importantly, CRISPR/Cas‐9 mediated excision of the C9orf72 repeat expansion reverses these phenotypes, confirming that the C9orf72 mutation is responsible for both cell‐autonomous astrocyte pathology and non‐cell autonomous motor neuron pathophysiology.

Modeling cell-autonomous motor neuron phenotypes in ALS using iPSCs

Amyotrophic lateral sclerosis (ALS) is an aggressive and uniformly fatal degenerative disease of the motor nervous system. In order to understand underlying disease mechanisms, researchers leverage a host of in vivo and in vitro models, including yeast, worms, flies, zebrafish, mice, and more recently, human induced pluripotent stem cells (iPSCs) derived from ALS patients. While mouse models have been the main workhorse of preclinical ALS research, the development of iPSCs provides a new opportunity to explore molecular phenotypes of ALS within human cells. Importantly, this technology enables modeling of both familial and sporadic ALS in the relevant human genetic backgrounds, as well as a personalized or targeted approach to therapy development. Harnessing these powerful tools requires addressing numerous challenges, including different variance components associated with iPSCs and motor neurons as well as concomitant limits of reductionist approaches. In order to overcome these obstacles, optimization of protocols and assays, confirmation of phenotype robustness at scale, and validation of findings in human tissue and genetics will cement the role for iPSC models as a valuable complement to animal models in ALS and more broadly among neurodegenerative diseases.

The Novel Small Molecule TRVA242 Stabilizes Neuromuscular Junction Defects in Multiple Animal Models of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a debilitating neurodegenerative disorder in which the neuromuscular junction progressively degenerates, leading to movement difficulties, paralysis, and eventually death. ALS is currently being treated by only two FDA-approved drugs with modest efficacy in slowing disease progression. Often, the translation of preclinical findings to bedside terminates prematurely as the evaluation of potential therapeutic compounds focuses on a single study or a single animal model. To circumscribe these issues, we screened 3,765 novel small molecule derivatives of pimozide, a recently identified repurposed neuroleptic for ALS, in Caenorhabditis elegans, confirmed the hits in zebrafish and validated the most active compounds in mouse genetic models. Out of the 27 small molecules identified from the high-throughput screen in worms, 4 were found to recover locomotor defects in C. elegans and genetic zebrafish models of ALS. TRVA242 was identified as the most potent compound as it significantly improved efficiency in rescuing locomotor, motorneuron, and neuromuscular junction synaptic deficits in a C. elegans TDP-43 model and in multiple zebrafish genetic (TDP-43, SOD1, and C9ORF72) models of ALS. The actions of TRVA242 were also conserved in a mammalian model as it also stabilized neuromuscular junction deficits in a mouse SOD1 model of ALS. Compounds such as TRVA242 therefore represent new potential therapeutics for the treatment of ALS.

Neuronal glucose metabolism is impaired while astrocytic TCA cycling is unaffected at symptomatic stages in the hSOD1G93A mouse model of amyotrophic lateral sclerosis

Although alterations in energy metabolism are known in ALS, the specific mechanisms leading to energy deficit are not understood. We measured metabolite levels derived from injected [1-¹³C]glucose and [1,2-¹³C]acetate (i.p.) in cerebral cortex and spinal cord extracts of wild type and hSOD1^G93A mice at onset and mid disease stages using high-pressure liquid chromatography, ¹H and ¹³C nuclear magnetic resonance spectroscopy. Levels of spinal and cortical CNS total lactate, [3-¹³C]lactate, total alanine and [3-¹³C]alanine, but not cortical glucose and [1-¹³C]glucose, were reduced mostly at mid stage indicating impaired glycolysis. The [1-¹³C]glucose-derived [4-¹³C]glutamate, [4-¹³C]glutamine and [2-¹³C]GABA amounts were diminished at mid stage in cortex and both time points in spinal cord, suggesting decreased [3-¹³C]pyruvate entry into the TCA cycle. Lack of changes in [1,2-¹³C]acetate-derived [4,5-¹³C]glutamate, [4,5-¹³C]glutamine and [1,2-¹³C]GABA levels indicate unchanged astrocytic ¹³C-acetate metabolism. Reduced levels of leucine, isoleucine and valine in CNS suggest compensatory breakdown to refill TCA cycle intermediate levels. Unlabelled, [2-¹³C] and [4-¹³C]GABA concentrations were decreased in spinal cord indicating that impaired glucose metabolism contributes to hyperexcitability and supporting the use of treatments which increase GABA amounts. In conclusion, CNS glucose metabolism is compromised, while astrocytic TCA cycling appears to be normal in the hSOD1^G93A mouse model at symptomatic disease stages.

Hyperexcitability precedes motoneuron loss in the Smn2B/- mouse model of spinal muscular atrophy

Spinal motoneuron dysfunction and loss are pathological hallmarks of the neuromuscular disease spinal muscular atrophy (SMA). Changes in motoneuron physiological function precede cell death, but how these alterations vary with disease severity and motoneuron maturational state is unknown. To address this question, we assessed the electrophysiology and morphology of spinal motoneurons of presymptomatic Smn^2B/- mice older than 1 wk of age and tracked the timing of motor unit loss in this model using motor unit number estimation (MUNE). In contrast to other commonly used SMA mouse models, Smn^2B/- mice exhibit more typical postnatal development until postnatal day (P)11 or 12 and have longer survival (~3 wk of age). We demonstrate that Smn^2B/- motoneuron hyperexcitability, marked by hyperpolarization of the threshold voltage for action potential firing, was present at P9-10 and preceded the loss of motor units. Using MUNE studies, we determined that motor unit loss in this mouse model occurred 2 wk after birth. Smn^2B/- motoneurons were also larger in size, which may reflect compensatory changes taking place during postnatal development. This work suggests that motoneuron hyperexcitability, marked by a reduced threshold for action potential firing, is a pathological change preceding motoneuron loss that is common to multiple models of severe SMA with different motoneuron maturational states. Our results indicate voltage-gated sodium channel activity may be altered in the disease process.NEW & NOTEWORTHY Changes in spinal motoneuron physiologic function precede cell death in spinal muscular atrophy (SMA), but how they vary with maturational state and disease severity remains unknown. This study characterized motoneuron and neuromuscular electrophysiology from the Smn^2B/- model of SMA. Motoneurons were hyperexcitable at postnatal day (P)9-10, and specific electrophysiological changes in Smn^2B/- motoneurons preceded functional motor unit loss at P14, as determined by motor unit number estimation studies.

Identification and therapeutic rescue of autophagosome and glutamate receptor defects in C9ORF72 and sporadic ALS neurons

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disease with diverse etiologies. Therefore, the identification of common disease mechanisms and therapeutics targeting these mechanisms could dramatically improve clinical outcomes. To this end, we developed induced motor neuron (iMN) models from C9ORF72 and sporadic ALS (sALS) patients to identify targets that are effective against these types of cases, which together comprise ~90% of patients. We find that iMNs from C9ORF72 and several sporadic ALS patients share two common defects - impaired autophagosome formation and the aberrant accumulation of glutamate receptors. Moreover, we show that an anticoagulation-deficient form of activated protein C, 3K3A-APC, rescues these defects in both C9ORF72 and sporadic ALS iMNs. As a result, 3K3A-APC treatment lowers C9ORF72 dipeptide repeat protein (DPR) levels, restores nuclear TDP-43 localization, and rescues the survival of both C9ORF72 and sporadic ALS iMNs. Importantly, 3K3A-APC also lowers glutamate receptor levels and rescues proteostasis in vivo in C9ORF72 gain- and loss-of-function mouse models. Thus, motor neurons from C9ORF72 and at least a subset of sporadic ALS patients share common, early defects in autophagosome formation and glutamate receptor homeostasis and a single therapeutic approach may be efficacious against these disease processes.

Pathophysiology and Diagnosis of ALS: Insights from Advances in Neurophysiological Techniques

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive and fatal neurodegenerative disorder of the motor neurons, characterized by focal onset of muscle weakness and incessant disease progression. While the presence of concomitant upper and lower motor neuron signs has been recognized as a pathognomonic feature of ALS, the pathogenic importance of upper motor neuron dysfunction has only been recently described. Specifically, transcranial magnetic stimulation (TMS) techniques have established cortical hyperexcitability as an important pathogenic mechanism in ALS, correlating with neurodegeneration and disease spread. Separately, ALS exhibits a heterogeneous clinical phenotype that may lead to misdiagnosis, particularly in the early stages of the disease process. Cortical hyperexcitability was shown to be a robust diagnostic biomarker if ALS, reliably differentiating ALS from neuromuscular mimicking disorders. The present review will provide an overview of key advances in the understanding of ALS pathophysiology and diagnosis, focusing on the importance of cortical hyperexcitability and its relationship to advances in genetic and molecular processes implicated in ALS pathogenesis.

β-catenin aggregation in models of ALS motor neurons: GSK3β inhibition effect and neuronal differentiation

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by motor neuron death. A 20% of familial ALS cases are associated with mutations in the gene coding for superoxide dismutase 1 (SOD1). The accumulation of abnormal aggregates of different proteins is a common feature in motor neurons of patients and transgenic ALS mice models, which are thought to contribute to disease pathogenesis. Developmental morphogens, such as the Wnt family, regulate numerous features of neuronal physiology in the adult brain and have been implicated in neurodegeneration. β-catenin is a central mediator of both, Wnt signaling activity and cell-cell interactions. We previously reported that the expression of mutant SOD1 in the NSC34 motor neuron cell line decreases basal Wnt pathway activity, which correlates with cytosolic β-catenin accumulation and impaired neuronal differentiation. In this work, we aimed a deeper characterization of β-catenin distribution in models of ALS motor neurons. We observed extensive accumulation of β-catenin supramolecular structures in motor neuron somas of pre-symptomatic mutant SOD1 mice. In cell-cell appositional zones of NSC34 cells expressing mutant SOD1, β-catenin displays a reduced co-distribution with E-cadherin accompanied by an increased association with the gap junction protein Connexin-43; these findings correlate with impaired intercellular adhesion and exacerbated cell coupling. Remarkably, pharmacological inhibition of the glycogen synthase kinase-3β (GSK3β) in both NSC34 cell lines reverted both, β-catenin aggregation and the adverse effects of mutant SOD1 expression on neuronal differentiation. Our findings suggest that early defects in β-catenin distribution could be an underlying factor affecting the onset of neurodegeneration in familial ALS.

Amyotrophic lateral sclerosis mutant TDP-43 may cause synaptic dysfunction through altered dendritic spine function

Altered cortical excitability and synapse dysfunction are early pathogenic events in amyotrophic lateral sclerosis (ALS) patients and animal models. Recent studies propose an important role for TAR DNA-binding protein 43 (TDP-43), the mislocalization and aggregation of which are key pathological features of ALS. However, the relationship between ALS-linked TDP-43 mutations, excitability and synaptic function is not fully understood. Here, we investigate the role of ALS-linked mutant TDP-43 in synapse formation by examining the morphological, immunocytochemical and excitability profile of transgenic mouse primary cortical pyramidal neurons that over-express human TDP-43^A315T. In TDP-43^A315T cortical neurons, dendritic spine density was significantly reduced compared to wild-type controls. TDP-43^A315T over-expression increased the total levels of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropinionic acid (AMPA) glutamate receptor subunit GluR1, yet the localization of GluR1 to the dendritic spine was reduced. These postsynaptic changes were coupled with a decrease in the amount of the presynaptic marker synaptophysin that colocalized with dendritic spines. Interestingly, action potential generation was reduced in TDP-43^A315T pyramidal neurons. This work reveals a crucial effect of the over-expression mutation TDP-43^A315T on the formation of synaptic structures and the recruitment of GluR1 to the synaptic membrane. This pathogenic effect may be mediated by cytoplasmic mislocalization of TDP-43^A315T. Loss of synaptic GluR1, and reduced excitability within pyramidal neurons, implicates hypoexcitability and attenuated synaptic function in the pathogenic decline of neuronal function in TDP-43-associated ALS. Further studies into the mechanisms underlying AMPA receptor-mediated excitability changes within the ALS cortical circuitry may yield novel therapeutic targets for treatment of this devastating disease.

Summary: Loss of synaptic GluR1, and reduced excitability within pyramidal neurons, implicates hypoexcitability and attenuated synaptic function in the pathogenic decline of neuronal function in TDP-43-associated ALS.

Neuromuscular junction abnormalities in a zebrafish loss-of-function model of TDP-43

Almost 90% of amyotrophic lateral sclerosis (ALS) cases are characterized by the presence of aggregates of insoluble, misfolded cytoplasmic TAR DNA binding protein of 43 kDa (TDP-43). Distal axonopathy with impaired neuromuscular junctions (NMJs) before motor neuron degeneration or clinical onset of symptoms has been hypothesized as an early pathology in ALS. However, synaptic defects at the NMJ caused by TDP-43 mutations have not been characterized. In this study, we examined a previously reported zebrafish line expressing the tardbp^Y220X/Y220X variant, which results in an unstable and degraded protein. These tardbp^-/- larvae, however, mature normally due to the upregulated expression of an alternative splice variant of the tardbp paralog tardbp-like, or tardbpl. We generated a mutant line with a CRISPR/Cas9-mediated 5-base pair deletion encompassing the ATG start codon of tardbpl and in-crossed these with tardbp^-/- mutants to obtain tardbp^-/- and tardbpl^-/- double mutants, herein referred to as hom/hom. We subsequently characterized morphological, coiling, locomotor, synaptic, and NMJ structural abnormalities in the hom/hom mutants and in their genotypic controls. We observed that hom/hom mutants displayed gross morphological defects, early lethality, reduced locomotor function, aberrant quantal transmission, and perturbed synapse architecture at the NMJ. We further employed pharmacological manipulations in an effort to rescue phenotypic defects and observed that tardbp^+/-; tardbpl^-/- (herein referred to as het/hom) mutants, but not hom/hom mutants, were sensitive to chronic treatments of BAY K 8644, an L-type calcium channel agonist. This result highlights the importance of partial vs. complete loss of allelic functions of TDP-43. NEW & NOTEWORTHY This study highlights the importance of partial vs. complete loss of allelic functions of TDP-43 in a zebrafish loss of function model, thus making it an attractive tool for drug screening approaches.

Reduced Excitability and Increased Neurite Complexity of Cortical Interneurons in a Familial Mouse Model of Amyotrophic Lateral Sclerosis

Cortical interneurons play a crucial role in regulating inhibitory-excitatory balance in brain circuits, filtering synaptic information and dictating the activity of pyramidal cells through the release of GABA. In the fatal motor neuron (MN) disease, amyotrophic lateral sclerosis (ALS), an imbalance between excitation and inhibition is an early event in the motor cortex, preceding the development of overt clinical symptoms. Patients with both sporadic and familial forms of the disease exhibit reduced cortical inhibition, including patients with mutations in the copper/zinc superoxide-dismutase-1 (SOD1) gene. In this study, we investigated the influence of the familial disease-causing hSOD1-G93A ALS mutation on cortical interneurons in neuronal networks. We performed whole-cell patch-clamp recordings and neurobiotin tracing from GFP positive interneurons in primary cortical cultures derived from Gad67-GFP::hSOD1^G93A mouse embryos. Targeted recordings revealed no overt differences in the passive properties of Gad67-GFP::hSOD1^G93A interneurons, however the peak outward current was significantly diminished and cells were less excitable compared to Gad67-GFP::WT controls. Post hoc neurite reconstruction identified a significantly increased morphological complexity of the Gad67-GFP::hSOD1^G93A interneuron neurite arbor compared to Gad67-GFP::WT controls. Our results from the SOD1 model suggest that cortical interneurons have electrophysiological and morphological alterations that could contribute to attenuated inhibitory function in the disease. Determining if these phenomena are driven by the network or represent intrinsic alteration of the interneuron may help explain the emergence of inhibitory susceptibility and ultimately disrupted excitability, in ALS.

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

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