Early intrinsic hyperexcitability does not contribute to motoneuron degeneration in amyotrophic lateral sclerosis

Dendritic morphology of motor neurons and interneurons within the compact, semicompact, and loose formations of the rat nucleus ambiguus

Introduction

Motor neurons (MNs) within the nucleus ambiguus innervate the skeletal muscles of the larynx, pharynx, and oesophagus. These muscles are activated during vocalisation and swallowing and must be coordinated with several respiratory and other behaviours. Despite many studies evaluating the projections and orientation of MNs within the nucleus ambiguus, there is no quantitative information regarding the dendritic arbours of MNs residing in the compact, and semicompact/loose formations of the nucleus ambiguus..

Methods

In female and male Fischer 344 rats, we evaluated MN number using Nissl staining, and MN and non-MN dendritic morphology using Golgi-Cox impregnation Brightfield imaging of transverse Nissl sections (15 μm) were taken to stereologically assess the number of nucleus ambiguus MNs within the compact and semicompact/loose formations. Pseudo-confocal imaging of Golgi-impregnated neurons within the nucleus ambiguus (sectioned transversely at 180 μm) was traced in 3D to determine dendritic arbourisation.

Results

We found a greater abundance of MNs within the compact than the semicompact/loose formations. Dendritic lengths, complexity, and convex hull surface areas were greatest in MNs of the semicompact/loose formation, with compact formation MNs being smaller. MNs from both regions were larger than non-MNs reconstructed within the nucleus ambiguus.

Conclusion

Adding HBLS to the diet could be a potentially effective strategy to improve horses' health.

Molecular, Morphological and Electrophysiological Differences between Alpha and Gamma Motoneurons with Special Reference to the Trigeminal Motor Nucleus of Rat

The muscle contraction during voluntary movement is controlled by activities of alpha- and gamma-motoneurons (αMNs and γMNs, respectively). In spite of the recent advances in research on molecular markers that can distinguish between αMNs and γMNs, electrophysiological membrane properties and firing patterns of γMNs have remained unknown, while those of αMNs have been clarified in detail. Because of the larger size of αMNs compared to γMNs, blindly or even visually recorded MNs were mostly αMNs, as demonstrated with molecular markers recently. Subsequently, the research on αMNs has made great progress in classifying their subtypes based on the molecular markers and electrophysiological membrane properties, whereas only a few studies demonstrated the electrophysiological membrane properties of γMNs. In this review article, we provide an overview of the recent advances in research on the classification of αMNs and γMNs based on molecular markers and electrophysiological membrane properties, and discuss their functional implication and significance in motor control.

How do we get from hyperexcitability to excitotoxicity in amyotrophic lateral sclerosis?

Amyotrophic lateral sclerosis is a devastating neurodegenerative disease that, at present, has no effective cure. Evidence of increased circulating glutamate and hyperexcitability of the motor cortex in patients with amyotrophic lateral sclerosis have provided an empirical support base for the 'dying forward' excitotoxicity hypothesis. The hypothesis postulates that increased activation of upper motor neurons spreads pathology to lower motor neurons in the spinal cord in the form of excessive glutamate release, which triggers excitotoxic processes. Many clinical trials have focused on therapies that target excitotoxicity via dampening neuronal activation, but not all are effective. As such, there is a growing tension between the rising tide of evidence for the 'dying forward' excitotoxicity hypothesis and the failure of therapies that target neuronal activation. One possible solution to these contradictory outcomes is that our interpretation of the current evidence requires revision in the context of appreciating the complexity of the nervous system and the limitations of the neurobiological assays we use to study it. In this review we provide an evaluation of evidence relevant to the 'dying forward' excitotoxicity hypothesis and by doing so, identify key gaps in our knowledge that need to be addressed. We hope to provide a road map from hyperexcitability to excitotoxicity so that we can better develop therapies for patients suffering from amyotrophic lateral sclerosis. We conclude that studies of upper motor neuron activity and their synaptic output will play a decisive role in the future of amyotrophic lateral sclerosis therapy.

Postnatal survival of phrenic motor neurons is promoted by BDNF/TrkB.FL signaling

The number of motor neurons (MNs) declines precipitously during the final trimester before birth. Thereafter, the number of MNs remains relatively stable, with their connections to skeletal muscle dependent on neurotrophins, including brain-derived neurotrophic factor (BDNF) signaling through its high-affinity full-length tropomyosin-related kinase receptor subtype B (TrkB.FL) receptor. As a genetic knockout of BDNF leads to extensive MN loss and postnatal death within 1-2 days after birth, we tested the hypothesis that postnatal inhibition of BDNF/TrkB.FL signaling is important for postnatal phrenic MN (PhMN) survival. In the present study, we used a 1NMPP1-sensitive TrkBF616A mutant mouse to evaluate the effects of inhibition of TrkB kinase activity on phrenic MN (PhMN) numbers and diaphragm muscle (DIAm) fiber cross-sectional area (CSA). Pups were exposed to 1NMPP1 or vehicle (DMSO) from birth to 21 days old (weaning) via the mother's ingestion in the drinking water. Following weaning, the right phrenic nerve was exposed in the neck and the proximal end dipped in a rhodamine solution to retrogradely label PhMNs. After 24 h, the cervical spinal cord and DIAm were excised. Labeled PhMNs were imaged using confocal microscopy, whereas DIAm strips were frozen at ∼1.5× resting length, cryosectioned, and stained with hematoxylin and eosin to assess CSA. We observed an ∼34% reduction in PhMN numbers and increased primary dendrite numbers in 1NMPP1-treated TrkBF616A mice. The distribution of PhMN size (somal surface area) DIAm fiber cross-sectional areas did not differ. We conclude that survival of PhMNs during early postnatal development is sensitive to BDNF/TrkB.FL signaling.NEW & NOTEWORTHY During early postnatal development, BDNF/TrkB signaling promotes PhMN survival. Inhibition of BDNF/TrkB signaling in early postnatal development does not impact PhMN size. Inhibition of BDNF/TrkB signaling in early postnatal development does not impact the number or CSA of DIAm fibers.

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

In neurological conditions affecting the brain, early-stage neural circuit adaption is key for long-term preservation of normal behaviour. We tested if motoneurons and respective microcircuits also adapt in the initial stages of disease progression 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, preceding muscle denervation and motoneuron death, recurrent inhibition mediated by Renshaw cells is reduced in half due to impaired quantal size associated with decreased glycine receptor density. Additionally, higher probability of release from proprioceptive Ia terminals leads to increased monosynaptic excitation to motoneurons. Surprisingly, the initial impairment in recurrent inhibition is not a widespread feature of inhibitory spinal circuits, such as group I inhibitory afferents, and is compensated at later stages of disease progression. We reveal that in disease conditions, spinal microcircuits undergo specific multiphasic homeostatic compensations to preserve force output.

Non-canonical adrenergic neuromodulation of motoneuron intrinsic excitability through β-receptors in wild-type and ALS mice

Altered neuronal excitability and synaptic inputs to motoneurons are part of the pathophysiology of Amyotrophic Lateral Sclerosis. The cAMP/PKA pathway regulates both of them but therapeutic interventions at this level are limited by the lack of knowledge about suitable pharmacological entry points. Here we used transcriptomics on microdissected and in situ motoneurons to reveal the modulation of PKA-coupled receptorome in SOD1(G93A) ALS mice, vs WT, demonstrating the dysregulation of multiple PKA-coupled GPCRs, in particular on vulnerable MNs, and the relative sparing of β-adrenergic receptors. In vivo MN electrophysiology showed that β2/β3 agonists acutely increase excitability, in particular the input/output relationship, demonstrating a non-canonical adrenergic neuromodulation mediated by β2/β3 receptors both in WT and SOD1 mice. The excitability increase corresponds to the upregulation of immediate-early gene expression and dysregulation of ion channels transcriptome. However the β2/β3 neuromodulation is submitted to a strong homeostasis, since a ten days delivery of β2/β3 agonists results in an abolition of the excitability increase. The homeostatic response is largely caused by a substantial downregulation of PKA-coupled GPCRs in MNs from WT and SOD1 mice. Thus, β-adrenergic receptors are physiologically involved in the regulation of MN excitability and transcriptomics, but, intriguingly, a strong homeostatic response is triggered upon chronic pharmacologic intervention.

Effect of size on expression of bistability in mouse spinal motoneurons

Bistability in spinal motoneurons supports tonic spike activity in the absence of excitatory drive. Earlier work in adult preparations suggested that smaller motoneurons innervating slow antigravity muscle fibers are more likely to generate bistability for postural maintenance. However, whether large motoneurons innervating fast-fatigable muscle fibers display bistability is still controversial. To address this, we examined the relationship between soma size and bistability in lumbar (L4-L5) ventrolateral α-motoneurons of choline acetyltransferase (ChAT)-green fluorescent protein (GFP) and Hb9-GFP mice during the first 4 wk of life. We found that as neuron size increases, the prevalence of bistability rises. Smaller α-motoneurons lack bistability, whereas larger fast α-motoneurons [matrix metalloproteinase-9 (MMP-9)+/Hb9+] with a soma area ≥ 400 µm2 exhibit significantly higher bistability. Ionic currents associated with bistability, including the persistent Nav1.6 current, the thermosensitive Trpm5 Ca2+-activated Na+ current, and the slowly inactivating Kv1.2 current, also scale with cell size. Serotonin evokes full bistability in large motoneurons with partial bistable properties but not in small motoneurons. Our study provides important insights into the neural mechanisms underlying bistability and how motoneuron size correlates with bistability in mice.NEW & NOTEWORTHY Bistability is not a common feature of all mouse spinal motoneurons. It is absent in small, slow motoneurons but present in most large, fast motoneurons. This difference results from differential expression of ionic currents that enable bistability, which are highly expressed in large motoneurons but small or absent in small motoneurons. These results support a possible role for fast motoneurons in maintenance of tonic posture in addition to their known roles in fast movements.

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.

Novel approaches to motoneuron disease/ALS treatment using non-invasive brain and spinal stimulation: IFCN handbook chapter

Non-invasive brain stimulation techniques have been exploited in motor neuron disease (MND) with multifold objectives: to support the diagnosis, to get insights in the pathophysiology of these disorders and, more recently, to slow down disease progression. In this review, we consider how neuromodulation can now be employed to treat MND, with specific attention to amyotrophic lateral sclerosis (ALS), the most common form with upper motoneuron (UMN) involvement, taking into account electrophysiological abnormalities revealed by human and animal studies that can be targeted by neuromodulation techniques. This review article encompasses repetitive transcranial magnetic stimulation methods (including low-frequency, high-frequency, and pattern stimulation paradigms), transcranial direct current stimulation as well as experimental findings with the newer approach of trans-spinal direct current stimulation. We also survey and discuss the trials that have been performed, and future perspectives.

M-type potassium currents differentially affect activation of motoneuron subtypes and tune recruitment gain

The size principle is a key mechanism governing the orderly recruitment of motor units and is believed to be dependent on passive properties of the constituent motoneurons. However, motoneurons are endowed with voltage-sensitive ion channels that create non-linearities in their input-output functions. Here we describe a role for the M-type potassium current, conducted by KCNQ channels, in the control of motoneuron recruitment in mice. Motoneurons were studied with whole-cell patch clamp electrophysiology in transverse spinal slices and identified based on delayed (fast) and immediate (slow) onsets of repetitive firing. M-currents were larger in delayed compared to immediate firing motoneurons, which was not reflected by variations in the presence of Kv7.2 or Kv7.3 subunits. Instead, a more depolarized spike threshold in delayed-firing motoneurons afforded a greater proportion of the total M-current to become activated within the subthreshold voltage range, which translated to a greater influence on their recruitment with little influence on their firing rates. Pharmacological activation of M-currents also influenced motoneuron recruitment at the population level, producing a rightward shift in the recruitment curve of monosynaptic reflexes within isolated mouse spinal cords. These results demonstrate a prominent role for M-type potassium currents in regulating the function of motor units, which occurs primarily through the differential control of motoneuron subtype recruitment. More generally, these findings highlight the importance of active properties mediated by voltage-sensitive ion channels in the differential control of motoneuron recruitment, which is a key mechanism for the gradation of muscle force. KEY POINTS: M-currents exert an inhibitory influence on spinal motor output. This inhibitory influence is exerted by controlling the recruitment, but not the firing rate, of high-threshold fast-like motoneurons, with limited influence on low-threshold slow-like motoneurons. Preferential control of fast motoneurons may be linked to a larger M-current that is activated within the subthreshold voltage range compared to slow motoneurons. Larger M-currents in fast compared to slow motoneurons are not accounted for by differences in Kv7.2 or Kv7.3 channel composition. The orderly recruitment of motoneuron subtypes is shaped by differences in the contribution of voltage-gated ion channels, including KCNQ channels. KCNQ channels may provide a target to dynamically modulate the recruitment gain across the motor pool and readily adjust movement vigour.

A size principle for bistability in mouse spinal motoneurons

Bistability in spinal motoneurons supports tonic spike activity in the absence of excitatory drive. Earlier work in adult preparations suggested that smaller motoneurons innervating slow antigravity muscle fibers are more likely to generate bistability for postural maintenance. However, whether large motoneurons innervating fast-fatigable muscle fibers display bistability related to postural tone is still controversial. To address this, we examined the relationship between soma size and bistability in lumbar ventrolateral α-motoneurons of ChAT-GFP and Hb9-GFP mice across different developmental stages: neonatal (P2-P7), young (P7-P14) and mature (P21-P25). We found that as neuron size increases, the prevalence of bistability rises. Smaller α-motoneurons lack bistability, while larger fast α-motoneurons (MMP-9 + /Hb9 + ) with a soma area ≥ 400µm 2 exhibit significantly higher bistability. Ionic currents associated with bistability, including the persistent Nav1.6 current, thermosensitive Trpm5 Ca 2+ -activated Na + current and the slowly inactivating Kv1.2 current, also scale with cell size. Serotonin evokes full bistability in large motoneurons with partial bistable properties, but not in small motoneurons. Our study provides important insights into the neural mechanisms underlying bistability and how motoneuron size dictates this process.

New and Noteworthy

Bistability is not a common feature of all mouse spinal motoneurons. It is absent in small, slow motoneurons but present in most large, fast motoneurons. This difference results from differential expression of ionic currents that enable bistability, which are highly expressed in large motoneurons but small or absent in small motoneurons. These results support a possible role for fast motoneurons in maintenance of tonic posture in addition to their known roles in fast movements.

Peptidergic modulation of motor neuron output via CART signaling at C bouton synapses

The intensity of muscle contraction, and therefore movement vigor, needs to be adaptable to enable complex motor behaviors. This can be achieved by adjusting the properties of motor neurons, which form the final common pathway for all motor output from the central nervous system. Here, we identify roles for a neuropeptide, cocaine- and amphetamine-regulated transcript (CART), in the control of movement vigor. We reveal distinct but parallel mechanisms by which CART and acetylcholine, both released at C bouton synapses on motor neurons, selectively amplify the output of subtypes of motor neurons that are recruited during intense movement. We find that mice with broad genetic deletion of CART or selective elimination of acetylcholine from C boutons exhibit deficits in behavioral tasks that require higher levels of motor output. Overall, these data uncover spinal modulatory mechanisms that control movement vigor to support movements that require a high degree of muscle force.

Integrative proteomics highlight presynaptic alterations and c-Jun misactivation as convergent pathomechanisms in ALS

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease mainly affecting upper and lower motoneurons. Several functionally heterogeneous genes have been associated with the familial form of this disorder (fALS), depicting an extremely complex pathogenic landscape. This heterogeneity has limited the identification of an effective therapy, and this bleak prognosis will only improve with a greater understanding of convergent disease mechanisms. Recent evidence from human post-mortem material and diverse model systems has highlighted the synapse as a crucial structure actively involved in disease progression, suggesting that synaptic aberrations might represent a shared pathological feature across the ALS spectrum. To test this hypothesis, we performed the first comprehensive analysis of the synaptic proteome from post-mortem spinal cord and human iPSC-derived motoneurons carrying mutations in the major ALS genes. This integrated approach highlighted perturbations in the molecular machinery controlling vesicle release as a shared pathomechanism in ALS. Mechanistically, phosphoproteomic analysis linked the presynaptic vesicular phenotype to an accumulation of cytotoxic protein aggregates and to the pro-apoptotic activation of the transcription factor c-Jun, providing detailed insights into the shared pathobiochemistry in ALS. Notably, sub-chronic treatment of our iPSC-derived motoneurons with the fatty acid docosahexaenoic acid exerted a neuroprotective effect by efficiently rescuing the alterations revealed by our multidisciplinary approach. Together, this study provides strong evidence for the central and convergent role played by the synaptic microenvironment within the ALS spinal cord and highlights a potential therapeutic target that counteracts degeneration in a heterogeneous cohort of human motoneuron cultures.

Neuronal branching is increasingly asymmetric near synapses, potentially enabling plasticity while minimizing energy dissipation and conduction time

Neurons' primary function is to encode and transmit information in the brain and body. The branching architecture of axons and dendrites must compute, respond and make decisions while obeying the rules of the substrate in which they are enmeshed. Thus, it is important to delineate and understand the principles that govern these branching patterns. Here, we present evidence that asymmetric branching is a key factor in understanding the functional properties of neurons. First, we derive novel predictions for asymmetric scaling exponents that encapsulate branching architecture associated with crucial principles such as conduction time, power minimization and material costs. We compare our predictions with extensive data extracted from images to associate specific principles with specific biophysical functions and cell types. Notably, we find that asymmetric branching models lead to predictions and empirical findings that correspond to different weightings of the importance of maximum, minimum or total path lengths from the soma to the synapses. These different path lengths quantitatively and qualitatively affect energy, time and materials. Moreover, we generally observe that higher degrees of asymmetric branching-potentially arising from extrinsic environmental cues and synaptic plasticity in response to activity-occur closer to the tips than the soma (cell body).

Early molecular layer interneuron hyperactivity triggers Purkinje neuron degeneration in SCA1

Toxic proteinaceous deposits and alterations in excitability and activity levels characterize vulnerable neuronal populations in neurodegenerative diseases. Using in vivo two-photon imaging in behaving spinocerebellar ataxia type 1 (Sca1) mice, wherein Purkinje neurons (PNs) degenerate, we identify an inhibitory circuit element (molecular layer interneurons [MLINs]) that becomes prematurely hyperexcitable, compromising sensorimotor signals in the cerebellum at early stages. Mutant MLINs express abnormally elevated parvalbumin, harbor high excitatory-to-inhibitory synaptic density, and display more numerous synaptic connections on PNs, indicating an excitation/inhibition imbalance. Chemogenetic inhibition of hyperexcitable MLINs normalizes parvalbumin expression and restores calcium signaling in Sca1 PNs. Chronic inhibition of mutant MLINs delayed PN degeneration, reduced pathology, and ameliorated motor deficits in Sca1 mice. Conserved proteomic signature of Sca1 MLINs, shared with human SCA1 interneurons, involved the higher expression of FRRS1L, implicated in AMPA receptor trafficking. We thus propose that circuit-level deficits upstream of PNs are one of the main disease triggers in SCA1.

SRF deletion results in earlier disease onset in a mouse model of amyotrophic lateral sclerosis

Changes in neuronal activity modulate the vulnerability of motoneurons (MNs) in neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). So far, the molecular basis of neuronal activity's impact in ALS is poorly understood. Herein, we investigated the impact of deleting the neuronal activity-stimulated transcription factor (TF) serum response factor (SRF) in MNs of SOD1G93A mice. SRF was present in vulnerable MMP9+ MNs. Ablation of SRF in MNs induced an earlier disease onset starting around 7-8 weeks after birth, as revealed by enhanced weight loss and decreased motor ability. This earlier disease onset in SRF-depleted MNs was accompanied by a mild elevation of neuroinflammation and neuromuscular synapse degeneration, whereas overall MN numbers and mortality were unaffected. In SRF-deficient mice, MNs showed impaired induction of autophagy-encoding genes, suggesting a potentially new SRF function in transcriptional regulation of autophagy. Complementary, constitutively active SRF-VP16 enhanced autophagy-encoding gene transcription and autophagy progression in cells. Furthermore, SRF-VP16 decreased ALS-associated aggregate induction. Chemogenetic modulation of neuronal activity uncovered SRF as having important TF-mediating activity-dependent effects, which might be beneficial to reduce ALS disease burden. Thus, our data identify SRF as a gene regulator connecting neuronal activity with the cellular autophagy program initiated in degenerating MNs.

Size-dependent differences in mitochondrial volume density in phrenic motor neurons

Neuromotor control of diaphragm muscle (DIAm) motor units is dependent on an orderly size-dependent recruitment of phrenic motor neurons (PhMNs). Slow (type S) and fast, fatigue resistant (type FR) DIAm motor units, which are frequently recruited to sustain ventilation, comprise smaller PhMNs that innervate type I and IIa DIAm fibers. More fatigable fast (type FF) motor units, which are infrequently recruited for higher force, expulsive behaviors, comprise larger PhMNs that innervate more type IIx/IIb DIAm fibers. We hypothesize that due to the more frequent activation and thus higher energy demand of type S and FR motor units, the mitochondrial volume density (MVD) of smaller PhMNs is greater compared with larger PhMNs. In eight adult (6 mo old) Fischer 344 rats, PhMNs were identified via intrapleural injection of Alexa488-conjugated cholera toxin B (CTB). Following retrograde CTB labeling, mitochondria in PhMNs were labeled by transdural infusion of MitoTracker Red. PhMNs and mitochondria were imaged using multichannel confocal microscopy using a ×60 oil objective. Following optical sectioning and three-dimensional (3-D) rendering, PhMNs and mitochondria were analyzed volumetrically using Nikon Elements software. Analysis of MVD in somal and dendritic compartments was stratified by PhMN somal surface area. Smaller PhMNs (likely S and FR units) had greater somal MVDs compared with larger PhMNs (likely FF units). By contrast, proximal dendrites or larger PhMNs had higher MVD compared with dendrites of smaller PhMNs. We conclude that more active smaller PhMNs have a higher mitochondrial volume density to support their higher energy demand in sustaining ventilation.NEW & NOTEWORTHY Type S and FR motor units, comprising smaller phrenic motor neurons (PhMNs) are regularly activated to perform indefatigable ventilatory requirements. By contrast, type FF motor units, comprising larger PhMNs, are infrequently activated to perform expulsive straining and airway defense maneuvers. This difference in activation history is mirrored in the mitochondrial volume density (MVD), with smaller PhMNs having higher MVD than larger PhMNs. In proximal dendrites, this trend was reversed, with larger PhMNs having higher MVD than smaller PhMNs, likely due to the maintenance requirements for the larger dendritic arbor of FF PhMNs.

Chemogenetic inhibition of TrkB signalling reduces phrenic motor neuron survival and size

Brain derived neurotrophic factor (BDNF) signalling through its high-affinity tropomyosin receptor kinase B (TrkB) is known to have potent effects on motor neuron survival and morphology during development and in neurodegenerative diseases. Here, we employed a novel 1NMPP1 sensitive TrkBF616 rat model to evaluate the effect of 14 days inhibition of TrkB signalling on phrenic motor neurons (PhMNs). Adult female and male TrkBF616 rats were divided into 1NMPP1 or vehicle treated groups. Three days prior to treatment, PhMNs in both groups were initially labeled via intrapleural injection of Alexa-Fluor-647 cholera toxin B (CTB). After 11 days of treatment, retrograde axonal uptake/transport was assessed by secondary labeling of PhMNs by intrapleural injection of Alexa-Fluor-488 CTB. After 14 days of treatment, the spinal cord was excised 100 μm thick spinal sections containing PhMNs were imaged using two-channel confocal microscopy. TrkB inhibition reduced the total number of PhMNs by ∼16 %, reduced the mean PhMN somal surface areas by ∼25 %, impaired CTB uptake 2.5-fold and reduced the estimated PhMN dendritic surface area by ∼38 %. We conclude that inhibition of TrkB signalling alone in adult TrkBF616 rats is sufficient to lead to PhMN loss, morphological degeneration and deficits in retrograde axonal uptake/transport.

Neuronal Branching is Increasingly Asymmetric Near Synapses, Potentially Enabling Plasticity While Minimizing Energy Dissipation and Conduction Time

Neurons' primary function is to encode and transmit information in the brain and body. The branching architecture of axons and dendrites must compute, respond, and make decisions while obeying the rules of the substrate in which they are enmeshed. Thus, it is important to delineate and understand the principles that govern these branching patterns. Here, we present evidence that asymmetric branching is a key factor in understanding the functional properties of neurons. First, we derive novel predictions for asymmetric scaling exponents that encapsulate branching architecture associated with crucial principles such as conduction time, power minimization, and material costs. We compare our predictions with extensive data extracted from images to associate specific principles with specific biophysical functions and cell types. Notably, we find that asymmetric branching models lead to predictions and empirical findings that correspond to different weightings of the importance of maximum, minimum, or total path lengths from the soma to the synapses. These different path lengths quantitatively and qualitatively affect energy, time, and materials. Moreover, we generally observe that higher degrees of asymmetric branching- potentially arising from extrinsic environmental cues and synaptic plasticity in response to activity- occur closer to the tips than the soma (cell body).

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.

Aging affects the number and morphological heterogeneity of rat phrenic motor neurons and phrenic motor axons

Diaphragm muscle (DIAm) motor units comprise a phrenic motor neuron (PhMN), the phrenic nerve and the muscle fibers innervated, with the size of PhMNs and axons characteristic of motor unit type. Smaller PhMNs and their axons comprise slow (type S) and fatigue-resistant (type FR) DIAm motor units, while larger PhMNs and their axons comprise more fatigable (type FF) motor units. With aging, we have shown a loss of larger PhMNs, consistent with selective atrophy of type IIx/IIb DIAm fibers and reduced maximum DIAm force. In the present study, we hypothesized that with aging there is a loss of larger myelinated phrenic α motor axons. Female and male young (6 months) and old (24 months) Fischer 344 rats were studied. PhMNs were retrogradely labeled by intrapleural injection of 488-conjugated CTB. The phrenic nerves were excised ~1 cm from the DIAm insertion and mounted in resin, and phrenic α motor axons were delineated based on size (i.e., >4 μm diameters). In older rats, the number of larger PhMNs and larger phrenic α motor axons were reduced. There were no differences in non-α axons. In addition, there was evidence of demyelination of larger phrenic α motor axons in older rats. Together, these findings are consistent with the selective age-related vulnerability of larger PhMNs and denervation of type FF motor units, which may underlie DIAm sarcopenia.

How axon and dendrite branching are guided by time, energy, and spatial constraints

Neurons are connected by complex branching processes-axons and dendrites-that process information for organisms to respond to their environment. Classifying neurons according to differences in structure or function is a fundamental part of neuroscience. Here, by constructing biophysical theory and testing against empirical measures of branching structure, we develop a general model that establishes a correspondence between neuron structure and function as mediated by principles such as time or power minimization for information processing as well as spatial constraints for forming connections. We test our predictions for radius scale factors against those extracted from neuronal images, measured for species that range from insects to whales, including data from light and electron microscopy studies. Notably, our findings reveal that the branching of axons and peripheral nervous system neurons is mainly determined by time minimization, while dendritic branching is determined by power minimization. Our model also predicts a quarter-power scaling relationship between conduction time delay and body size.

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.

In vitro longitudinal lumbar spinal cord preparations to study sensory and recurrent motor microcircuits of juvenile mice

In vitro spinal cord preparations have been extensively used to study microcircuits involved in the control of movement. By allowing precise control of experimental conditions coupled with state-of-the-art genetics, imaging, and electrophysiological techniques, isolated spinal cords from mice have been an essential tool in detailing the identity, connectivity, and function of spinal networks. The majority of the research has arisen from in vitro spinal cords of neonatal mice, which are still undergoing important postnatal maturation. Studies from adults have been attempted in transverse slices, however, these have been quite challenging due to the poor motoneuron accessibility and viability, as well as the extensive damage to the motoneuron dendritic trees. In this work, we describe two types of coronal spinal cord preparations with either the ventral or the dorsal horn ablated, obtained from mice of different postnatal ages, spanning from preweaned to 1 mo old. These semi-intact preparations allow recordings of sensory-afferent and motor-efferent responses from lumbar motoneurons using whole cell patch-clamp electrophysiology. We provide details of the slicing procedure and discuss the feasibility of whole cell recordings. The in vitro dorsal and ventral horn-ablated spinal cord preparations described here are a useful tool to study spinal motor circuits in young mice that have reached the adult stages of locomotor development.NEW & NOTEWORTHY In the past 20 years, most of the research into the mammalian spinal circuitry has been limited to in vitro preparations from embryonic and neonatal mice. We describe two in vitro longitudinal lumbar spinal cord preparations from juvenile mice that allow the study of motoneuron properties and respective afferent or efferent spinal circuits through whole cell patch clamp. These preparations will be useful to those interested in the study of microcircuits at mature stages of motor development.

Recent Updates on the Genetics of Amyotrophic Lateral Sclerosis and Frontotemporal Dementia

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) primarily affect the motor and frontotemporal areas of the brain, respectively. These disorders share clinical, genetic, and pathological similarities, and approximately 10-15% of ALS-FTD cases are considered to be multisystemic. ALS-FTD overlaps have been linked to families carrying an expansion in the intron of C9orf72 along with inclusions of TDP-43 in the brain. Other overlapping genes (VCP, FUS, SQSTM1, TBK1, CHCHD10) are also involved in similar functions that include RNA processing, autophagy, proteasome response, protein aggregation, and intracellular trafficking. Recent advances in genome sequencing have identified new genes that are involved in these disorders (TBK1, CCNF, GLT8D1, KIF5A, NEK1, C21orf2, TBP, CTSF, MFSD8, DNAJC7). Additional risk factors and modifiers have been also identified in genome-wide association studies and array-based studies. However, the newly identified genes show higher disease frequencies in combination with known genes that are implicated in pathogenesis, thus indicating probable digenetic/polygenic inheritance models, along with epistatic interactions. Studies suggest that these genes play a pleiotropic effect on ALS-FTD and other diseases such as Alzheimer's disease, Ataxia, and Parkinsonism. Besides, there have been numerous improvements in the genotype-phenotype correlations as well as clinical trials on stem cell and gene-based therapies. This review discusses the possible genetic models of ALS and FTD, the latest therapeutics, and signaling pathways involved in ALS-FTD.

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.

Transient increase in recurrent inhibition in amyotrophic lateral sclerosis as a putative protection from neurodegeneration

AIM

Adaptive mechanisms in spinal circuits are likely involved in homeostatic responses to maintain motor output in amyotrophic lateral sclerosis. Given the role of Renshaw cells in regulating the motoneuron input/output gain, we investigated the modulation of heteronymous recurrent inhibition.

METHODS

Electrical stimulations were used to activate recurrent collaterals resulting in the Hoffmann reflex depression. Inhibitions from soleus motor axons to quadriceps motoneurons, and vice versa, were tested in 38 patients and matched group of 42 controls.

RESULTS

Compared with controls, the mean depression of quadriceps reflex was larger in patients, while that of soleus was smaller, suggesting that heteronymous recurrent inhibition was enhanced in quadriceps but reduced in soleus. The modulation of recurrent inhibition was linked to the size of maximal direct motor response and lower limb dysfunctions, suggesting a significant relationship with the integrity of the target motoneuron pool and functional abilities. No significant link was found between the integrity of motor axons activating Renshaw cells and the level of inhibition. Enhanced inhibition was particularly observed in patients within the first year after symptom onset and with slow progression of lower limb dysfunctions. Normal or reduced inhibitions were mainly observed in patients with motor weakness first in lower limbs and greater dysfunctions in lower limbs.

CONCLUSION

We provide the first evidence for enhanced recurrent inhibition and speculate that Renshaw cells might have transient protective role on motoneuron by counteracting hyperexcitability at early stages. Several mechanisms likely participate including cortical influence on Renshaw cell and reinnervation by slow motoneurons.

Electrical Properties of Adult Mammalian Motoneurons

Motoneurons are the 'final common path' between the central nervous system (that intends, selects, commands, and organises movement) and muscles (that produce the behaviour). Motoneurons are not passive relays, but rather integrate synaptic activity to appropriately tune output (spike trains) and therefore the production of muscle force. In this chapter, we focus on studies of mammalian motoneurons, describing their heterogeneity whilst providing a brief historical account of motoneuron recording techniques. Next, we describe adult motoneurons in terms of their passive, transition, and active (repetitive firing) properties. We then discuss modulation of these properties by somatic (C-boutons) and dendritic (persistent inward currents) mechanisms. Finally, we briefly describe select studies of human motor unit physiology and relate them to findings from animal preparations discussed earlier in the chapter. This interphyletic approach to the study of motoneuron physiology is crucial to progress understanding of how these diverse neurons translate intention into behaviour.

Synaptic Projections of Motoneurons Within the Spinal Cord

Motoneurons have long been considered as the final common pathway of the nervous system, transmitting the neural impulses that are transduced into action.While many studies have focussed on the inputs that motoneurons receive from local circuits within the spinal cord and from other parts of the CNS, relatively few have investigated the targets of local axonal projections from motoneurons themselves, with the notable exception of those contacting Renshaw cells or other motoneurons.Recent research has not only characterised the detailed features of the excitatory connections between motoneurons and Renshaw cells but has also established that Renshaw cells are not the only target of motoneurons axons within the spinal cord. Motoneurons also form synaptic contacts with other motoneurons as well as with a subset of ventrally located V3 interneurons. These findings indicate that motoneurons cannot be simply viewed as the last relay station delivering the command drive to muscles, but perform an active role in the generation and modulation of motor patterns.

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.

Recruitment of Motoneurons

Beginning about half a century ago, the rules that determine how motor units are recruited during movement have been deduced. These classical experiments led to the formulation of the 'size principle'. It is now clear that motoneuronal size is not the only indicator of recruitment order. In fact, motoneuronal passive, active and synaptic conductances are carefully tuned to achieve sequential recruitment. More recent studies, over the last decade or so, show that the premotor circuitry is also functionally specialized and differentially recruited. Modular sub networks of interneurons and their post-synaptic motoneurons have been shown to drive movements with varying intensities. In addition, these modular networks are under the influence of neuromodulators, which are capable of acting upon multiple motor and premotor targets, thereby altering behavioral outcomes. We discuss the recruitment patterns of motoneurons in light of these new and exciting studies.

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.

Diversity of Mammalian Motoneurons and Motor Units

Although they share the common function of controlling muscle fiber contraction, spinal motoneurons display a remarkable diversity. Alpha-motoneurons are the "final common pathway", which relay all the information from spinal and supraspinal centers and allow the organism to interact with the outside world by controlling the contraction of muscle fibers in the muscles. On the other hand, gamma-motoneurons are specialized motoneurons that do not generate force and instead specifically innervate muscle fibers inside muscle spindles, which are proprioceptive organs embedded in the muscles. Beta-motoneurons are hybrid motoneurons that innervate both extrafusal and intrafusal muscle fibers. Even among alpha-motoneurons, there exists an exquisite diversity in terms of motoneuron electrical and molecular properties, physiological and structural properties of their neuromuscular junctions, and molecular and contractile properties of the innervated muscle fibers. This diversity, across species, across muscles, and across muscle fibers in a given muscle, underlie the vast repertoire of movements that one individual can perform.

The strength of corticomotoneuronal drive underlies ALS split phenotypes and reflects early upper motor neuron dysfunction

BACKGROUND

Split phenotypes, (split hand, elbow, leg, and foot), are probably unique to ALS, and are characterized by having a shared peripheral input of both affected and unaffected muscles. This implies an anatomical origin rostral to the spinal cord, primarily within the cerebral cortex. Therefore, split phenotypes are a potential marker of ALS upper motor neuron pathology. However, to date, reports documenting upper motor neuron dysfunction in split phenotypes have been limited to using transcranial magnetic stimulation and cortical threshold tracking techniques. Here, we consider several other potential methodologies that could confirm a primary upper motor neuron pathology in split phenotypes.

METHODS

We review the potential of: 1. measuring the compound excitatory post-synaptic potential recorded from a single activated motor unit, 2. cortical-muscular coherence, and 3. new advanced modalities of neuroimaging (high-resolution imaging protocols, ultra-high field MRI platforms [7T], and novel Non-Gaussian diffusion models).

CONCLUSIONS

We propose that muscles involved in split phenotypes are those functionally involved in the human motor repertoire used particularly in complex activities. Their anterior horn cells receive the strongest corticomotoneuronal input. This is also true of the weakest muscles that are the earliest to be affected in ALS. Descriptions of split hand in non-ALS cases and proposals that peripheral nerve or muscle dysfunction may be causative are contentious. Only a few carefully controlled cases of each form of split phenotype, using upper motor neuron directed methodologies, are necessary to prove our postulate.

Axon initial segment geometry in relation to motoneuron excitability

The axon initial segment (AIS) responsible for action potential initiation is a dynamic structure that varies and changes together with neuronal excitability. Like other neuron types, alpha motoneurons in the mammalian spinal cord express heterogeneity and plasticity in AIS geometry, including length (AIS_l) and distance from soma (AIS_d). The present study aimed to establish the relationship of AIS geometry with a measure of intrinsic excitability, rheobase current, that varies by 20-fold or more among normal motoneurons. We began by determining whether AIS length or distance differed for motoneurons in motor pools that exhibit different activity profiles. Motoneurons sampled from the medial gastrocnemius (MG) motor pool exhibited values for average AIS_d that were significantly greater than that for motoneurons from the soleus (SOL) motor pool, which is more readily recruited in low-level activities. Next, we tested whether AIS_d covaried with intrinsic excitability of individual motoneurons. In anesthetized rats, we measured rheobase current intracellularly from MG motoneurons in vivo before labeling them for immunohistochemical study of AIS structure. For 16 motoneurons sampled from the MG motor pool, this combinatory approach revealed that AIS_d, but not AIS_l, was significantly related to rheobase, as AIS tended to be located further from the soma on motoneurons that were less excitable. Although a causal relation with excitability seems unlikely, AIS_d falls among a constellation of properties related to the recruitability of motor units and their parent motoneurons.

Maturation of persistent and hyperpolarization-activated inward currents shapes the differential activation of motoneuron subtypes during postnatal development

The size principle underlies the orderly recruitment of motor units; however, motoneuron size is a poor predictor of recruitment amongst functionally defined motoneuron subtypes. Whilst intrinsic properties are key regulators of motoneuron recruitment, the underlying currents involved are not well defined. Whole-cell patch-clamp electrophysiology was deployed to study intrinsic properties, and the underlying currents, that contribute to the differential activation of delayed and immediate firing motoneuron subtypes. Motoneurons were studied during the first three postnatal weeks in mice to identify key properties that contribute to rheobase and may be important to establish orderly recruitment. We find that delayed and immediate firing motoneurons are functionally homogeneous during the first postnatal week and are activated based on size, irrespective of subtype. The rheobase of motoneuron subtypes becomes staggered during the second postnatal week, which coincides with the differential maturation of passive and active properties, particularly persistent inward currents. Rheobase of delayed firing motoneurons increases further in the third postnatal week due to the development of a prominent resting hyperpolarization-activated inward current. Our results suggest that motoneuron recruitment is multifactorial, with recruitment order established during postnatal development through the differential maturation of passive properties and sequential integration of persistent and hyperpolarization-activated inward currents.

Targeting autotaxin impacts disease advance in the SOD1-G93A mouse model of amyotrophic lateral sclerosis

A preclinical strategy to broaden the search of potentially effective treatments in amyotrophic lateral sclerosis (ALS) relies on identifying factors controlling motor neuron (MN) excitability. These partners might be part of still unknown pathogenic pathways and/or useful for the design of new interventions to affect disease progression. In this framework, the bioactive membrane‐derived phospholipid lysophosphatidic acid (LPA) affects MN excitability through LPA receptor 1 (LPA₁). Furthermore, LPA₁ knockdown is neuroprotective in transgenic ALS SOD1‐G93A mice. On this basis, we raised the hypothesis that the major LPA‐synthesizing ectoenzyme, autotaxin (ATX), regulates MN excitability and is a potential target to modulate disease development in ALS mice. We show here that PF‐8380, a specific ATX inhibitor, reduced intrinsic membrane excitability (IME) of hypoglossal MNs in brainstem slices, supporting that baseline ATX activity regulates MN IME. PF‐8380‐induced alterations were prevented by a small‐interfering RNA directed against mRNA for lpa₁. These outcomes support that impact of ATX‐originated lysophospholipids on MN IME engages, at least, the G‐protein‐coupled receptor LPA₁. Interestingly, mRNA_atx levels increased in the spinal cord of pre‐symptomatic (1–2 months old) SOD1‐G93A mice, thus preceding MN loss. The rise in transcripts levels also occurred in cultured spinal cord MNs from SOD1‐G93A embryos, suggesting that mRNA_atx upregulation in MNs is an etiopathogenic event in the ALS cell model. Remarkably, chronic administration in the drinking water of the orally bioavailable ATX inhibitor PF‐8380 delayed MN loss, motor deterioration and prolonged life span in ALS mice. Treatment also led to a reduction in LPA₁‐immunoreactive patches in transgenic animals mostly in MNs. These outcomes support that neuroprotective effects of interfering with ATX in SOD1‐G93A mice rely, at least in part, on LPA₁ knockdown in MNs. Therefore, we propose ATX as a potential target and/or a biomarker in ALS and highlight ATX inhibitors as reasonable tools with therapeutic usefulness for this lethal pathology.

Synaptic disruption and CREB-regulated transcription are restored by K+ channel blockers in ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease, which is still missing effective therapeutic strategies. Although manipulation of neuronal excitability has been tested in murine and human ALS models, it is still under debate whether neuronal activity might represent a valid target for efficient therapies. In this study, we exploited a combination of transcriptomics, proteomics, optogenetics and pharmacological approaches to investigate the activity-related pathological features of iPSC-derived C9orf72-mutant motoneurons (MN). We found that human ALSC9orf72 MN are characterized by accumulation of aberrant aggresomes, reduced expression of synaptic genes, loss of synaptic contacts and a dynamic "malactivation" of the transcription factor CREB. A similar phenotype was also found in TBK1-mutant MN and upon overexpression of poly(GA) aggregates in primary neurons, indicating a strong convergence of pathological phenotypes on synaptic dysregulation. Notably, these alterations, along with neuronal survival, could be rescued by treating ALS-related neurons with the K+ channel blockers Apamin and XE991, which, respectively, target the SK and the Kv7 channels. Thus, our study shows that restoring the activity-dependent transcriptional programme and synaptic composition exerts a neuroprotective effect on ALS disease progression.

Cortical and Striatal Electroencephalograms and Apomorphine Effects in the FUS Mouse Model of Amyotrophic Lateral Sclerosis

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is characterized by degeneration of motor neurons resulting in muscle atrophy. In contrast to the lower motor neurons, the role of upper (cortical) neurons in ALS is yet unclear. Maturation of locomotor networks is supported by dopaminergic (DA) projections from substantia nigra to the spinal cord and striatum.

OBJECTIVE

To examine the contribution of DA mediation in the striatum-cortex networks in ALS progression.

METHODS

We studied electroencephalogram (EEG) from striatal putamen (Pt) and primary motor cortex (M1) in ΔFUS(1-359)-transgenic (Tg) mice, a model of ALS. EEG from M1 and Pt were recorded in freely moving young (2-month-old) and older (5-month-old) Tg and non-transgenic (nTg) mice. EEG spectra were analyzed for 30 min before and for 60 min after systemic injection of a DA mimetic, apomorphine (APO), and saline.

RESULTS

In young Tg versus nTg mice, baseline EEG spectra in M1 were comparable, whereas in Pt, beta activity in Tg mice was enhanced. In older Tg versus nTg mice, beta dominated in EEG from both M1 and Pt, whereas theta and delta 2 activities were reduced. In younger Tg versus nTg mice, APO increased theta and decreased beta 2 predominantly in M1. In older mice, APO effects in these frequency bands were inversed and accompanied by enhanced delta 2 and attenuated alpha in Tg versus nTg mice.

CONCLUSION

We suggest that revealed EEG modifications in ΔFUS(1-359)-transgenic mice are associated with early alterations in the striatum-cortex interrelations and DA transmission followed by adaptive intracerebral transformations.

Cortical hyperexcitability: Diagnostic and pathogenic biomarker of ALS

Cortical hyperexcitability is an early and intrinsic feature of both sporadic and familial forms of amyotrophic lateral sclerosis (ALS).. Importantly, cortical hyperexcitability appears to be associated with motor neuron degeneration, possibly via an anterograde glutamate-mediated excitotoxic process, thereby forming a pathogenic basis for ALS. The presence of cortical hyperexcitability in ALS patients may be readily determined by transcranial magnetic stimulation (TMS), a neurophysiological tool that provides a non-invasive and painless method for assessing cortical function. Utilising the threshold tracking TMS technique, cortical hyperexcitability has been established as a robust diagnostic biomarker that distinguished ALS from mimicking disorders at early stages of the disease process. The present review discusses the pathophysiological and diagnostic utility of cortical hyperexcitability in ALS.

Human amyotrophic lateral sclerosis excitability phenotype screen: Target discovery and validation

Drug development is hampered by poor target selection. Phenotypic screens using neurons differentiated from patient stem cells offer the possibility to validate known and discover novel disease targets in an unbiased fashion. To identify targets for managing hyperexcitability, a pathological feature of amyotrophic lateral sclerosis (ALS), we design a multi-step screening funnel using patient-derived motor neurons. High-content live cell imaging is used to evaluate neuronal excitability, and from a screen against a chemogenomic library of 2,899 target-annotated compounds, 67 reduce the hyperexcitability of ALS motor neurons carrying the SOD1(A4V) mutation, without cytotoxicity. Bioinformatic deconvolution identifies 13 targets that modulate motor neuron excitability, including two known ALS excitability modulators, AMPA receptors and Kv7.2/3 ion channels, constituting target validation. We also identify D2 dopamine receptors as modulators of ALS motor neuron excitability. This screen demonstrates the power of human disease cell-based phenotypic screens for identifying clinically relevant targets for neurological disorders.

Motor neuron hyperexcitability is observed in both ALS patients and their iPSC-derived neurons. Combining a high-content live imaging excitability phenotypic assay, high-throughput screening against a cross-annotated chemogenomic library, and bioinformatic enrichment analysis, Huang et al. identify targets modulating the hyperexcitability of ALS patient-derived motor neurons in an unbiased manner.

Locomotor deficits in a mouse model of ALS are paralleled by loss of V1-interneuron connections onto fast motor neurons

ALS is characterized by progressive inability to execute movements. Motor neurons innervating fast-twitch muscle-fibers preferentially degenerate. The reason for this differential vulnerability and its consequences on motor output is not known. Here, we uncover that fast motor neurons receive stronger inhibitory synaptic inputs than slow motor neurons, and disease progression in the SOD1G93A mouse model leads to specific loss of inhibitory synapses onto fast motor neurons. Inhibitory V1 interneurons show similar innervation pattern and loss of synapses. Moreover, from postnatal day 63, there is a loss of V1 interneurons in the SOD1G93A mouse. The V1 interneuron degeneration appears before motor neuron death and is paralleled by the development of a specific locomotor deficit affecting speed and limb coordination. This distinct ALS-induced locomotor deficit is phenocopied in wild-type mice but not in SOD1G93A mice after appearing of the locomotor phenotype when V1 spinal interneurons are silenced. Our study identifies a potential source of non-autonomous motor neuronal vulnerability in ALS and links ALS-induced changes in locomotor phenotype to inhibitory V1-interneurons.

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.

Estimating the effects of slicing on the electrophysiological properties of spinal motoneurons under normal and disease conditions

Although slice recordings from spinal motoneurons (MNs) are being widely used, the effects of slicing on the measured MN electrical properties under normal and disease conditions have not been assessed. Using high-fidelity cell models of neonatal wild-type (WT) and superoxide dismutase-1 (SOD) cells, we examined the effects of slice thickness, soma position within the slice, and slice orientation to estimate the error induced in measured MN electrical properties from spinal slices. Our results show that most MN electrical properties are not adversely affected by slicing, except for cell time constant, cell capacitance, and Ca²⁺ persistent inward current (PIC), which all exhibited large errors, regardless of the slice condition. Among the examined factors, soma position within the slice appears to be the strongest factor in influencing the magnitude of error in measured MN electrical properties. Transverse slices appear to have the least impact on measured MN electrical properties. Surprisingly, and despite their anatomical enlargement, we found that G85R-SOD MNs experience similar error in their measured electrical properties to those of WT MNs, but their errors are more sensitive to the soma position within the slice than WT MNs. Unless in thick and symmetrical slices, slicing appears to reduce motoneuron type differences. Accordingly, slice studies should attempt to record from MNs at the slice center to avoid large and inconsistent errors in measured cell properties and have valid cell measurements' comparisons. Our results, therefore, offer information that would enhance the rigor of MN electrophysiological data measured from the slice preparation under normal and disease conditions.NEW & NOTEWORTHY Although slice recordings from motoneurons are being widely used, the effects of slicing on the measured motoneuron electrical properties under normal and disease conditions have not been assessed. Using high-fidelity cell models of neonatal WT and SOD cells, we examined the effects of slice thickness, soma position within the slice, and slice orientation. Our results offer information that enhances the rigor of MN electrophysiological data measured from the slice preparation under normal and disease conditions.

Synaptic restoration by cAMP/PKA drives activity-dependent neuroprotection to motoneurons in ALS

Excessive excitation is hypothesized to cause motoneuron (MN) degeneration in amyotrophic lateral sclerosis (ALS), but actual proof of hyperexcitation in vivo is missing, and trials based on this concept have failed. We demonstrate, by in vivo single-MN electrophysiology, that, contrary to expectations, excitatory responses evoked by sensory and brainstem inputs are reduced in MNs of presymptomatic mutSOD1 mice. This impairment correlates with disrupted postsynaptic clustering of Homer1b, Shank, and AMPAR subunits. Synaptic restoration can be achieved by activation of the cAMP/PKA pathway, by either intracellular injection of cAMP or DREADD-Gs stimulation. Furthermore, we reveal, through independent control of signaling and excitability allowed by multiplexed DREADD/PSAM chemogenetics, that PKA-induced restoration of synapses triggers an excitation-dependent decrease in misfolded SOD1 burden and autophagy overload. In turn, increased MN excitability contributes to restoring synaptic structures. Thus, the decrease of excitation to MN is an early but reversible event in ALS. Failure of the postsynaptic site, rather than hyperexcitation, drives disease pathobiochemistry.

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.

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

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