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.

Physical Therapy Intervention Effects on Alteration of Spinal Excitability in Patients With Chronic Ankle Instability: A Systematic Review and Meta-analysis.

CONTEXT: Chronic ankle instability (CAI) is a common injury in athletes. Different forms of physical therapy have been applied to the population with CAI to assess their impact on spinal excitability. OBJECTIVE: The purpose of this systematic review and meta-analysis was to investigate the effectiveness of various physical therapy interventions on the alteration of spinal excitability in patients with CAI. DATA SOURCES: Four databases (EMBASE, MEDLINE, Cochrane CENTRAL, and Scopus) were searched from inception to November 2022. STUDY SELECTION: A total of 253 studies were obtained and screened; 11 studies on the effects of physical therapy intervention on the alteration of spinal excitability in patients with CAI were identified for meta-analysis. STUDY DESIGN: Systematic review and meta-analysis. LEVEL OF EVIDENCE: Level 3a. DATA EXTRACTION: A total of 11 studies that included the maximal Hoffmann reflex normalized by the maximal muscle response (H/M ratio) in the peroneus longus and soleus muscles were extracted and summarized. The quality of the studies was assessed using the PEDro scale. RESULTS: The extracted studies had an average PEDro score of 4.7 ± 1.4, indicating that most of them had fair-to-good quality. The physical therapy interventions included cryotherapy, taping, mobilization, proprioceptive training, and dry needling. The overall effects showed that the H/M ratios of the peroneus longus (P = 0.44, I2 = 0%) and soleus (P = 0.56,I2 = 22%) muscles were not changed by physical therapy in patients with CAI. CONCLUSION: The meta-analysis indicated that physical therapy interventions such as cryotherapy, taping, mobilization, proprioceptive training, and dry needling do not alter the spinal excitability in patients with CAI. Given that only 1 study reported ineffective changes in spinal excitability with dry needling, more research is essential to establish and validate its efficacy. PROSPERO REGISTRATION: CRD42022372998.

Mutual oligosynaptic inhibition of group Ia afferents between the anterior and posterior parts of the deltoid in humans.

The anterior (DA) and posterior parts of the deltoid (DP) show alternating contraction during shoulder flexion and extension movements. It is expected that an inhibitory spinal reflex between the DA and DP exists. In this study, spinal reflexes between the DA and DP were examined in healthy human subjects using post-stimulus time histogram (PSTH) and electromyogram averaging (EMG-A). Electrical conditioning stimulation was delivered to the axillary nerve branch that innervates the DA (DA nerve) and DP (DP nerve) with the intensity below the motor threshold. In the PSTH study, the stimulation to the DA and DP nerves inhibited (decrease in the firing probability) 31 of 54 DA motor units and 31 of 51 DP motor units. The inhibition was not provoked by cutaneous stimulation. The central synaptic delay of the inhibition between the DA and DP nerves was 1.5 ± 0.5 ms and 1.4 ± 0.4 ms (mean ± SD) longer than those of the homonymous facilitation of the DA and DP, respectively. In the EMG-A study, conditioning stimulation to the DA and DP nerves inhibited the rectified and averaged EMG of the DP and DA, respectively. The inhibition diminished with tonic vibration stimulation to the DA and DP and recovered 20-30 min after vibration removal. These findings suggest that oligo(di or tri)-synaptic inhibition mediated by group Ia afferents between the DA and DP exists in humans.

From data to discovery: AI-guided analysis of disease-relevant molecules in spinal muscular atrophy (SMA).

Spinal Muscular Atrophy is caused by partial loss of survival of motoneuron (SMN) protein expression. The numerous interaction partners and mechanisms influenced by SMN loss result in a complex disease. Current treatments restore SMN protein levels to a certain extent, but do not cure all symptoms. The prolonged survival of patients creates an increasing need for a better understanding of SMA. Although many SMN-protein interactions, dysregulated pathways, and organ phenotypes are known, the connections among them remain largely unexplored. Monogenic diseases are ideal examples for the exploration of cause-and-effect relationships to create a network describing the disease-context. Machine learning tools can utilize such knowledge to analyze similarities between disease-relevant molecules and molecules not described in the disease so far. We used an artificial intelligence-based algorithm to predict new genes of interest. The transcriptional regulation of 8 out of 13 molecules selected from the predicted set were successfully validated in an SMA mouse model. This bioinformatic approach, using the given experimental knowledge for relevance predictions, enhances efficient targeted research in SMA and potentially in other disease settings.

Motor unit discharge rate modulation during isometric contractions to failure is intensity- and modality-dependent.

The physiological mechanisms determining the progressive decline in the maximal muscle torque production capacity during isometric contractions to task failure are known to depend on task demands. Task-specificity of the associated adjustments in motor unit discharge rate (MUDR), however, remains unclear. This study examined MUDR adjustments during different submaximal isometric knee extension tasks to failure. Participants performed a sustained and an intermittent task at 20% and 50% of maximal voluntary torque (MVT), respectively (Experiment 1). High-density surface EMG signals were recorded from vastus lateralis (VL) and medialis (VM) and decomposed into individual MU discharge timings, with the identified MUs tracked from recruitment to task failure. MUDR was quantified and normalised to intervals of 10% of contraction time (CT). MUDR of both muscles exhibited distinct modulation patterns in each task. During the 20% MVT sustained task, MUDR decreased until ∼50% CT, after which it gradually returned to baseline. Conversely, during the 50% MVT intermittent task, MUDR remained stable until ∼40-50% CT, after which it started to continually increase until task failure. To explore the effect of contraction intensity on the observed patterns, VL and VM MUDR was quantified during sustained contractions at 30% and 50% MVT (Experiment 2). During the 30% MVT sustained task, MUDR remained stable until ∼80-90% CT in both muscles, after which it continually increased until task failure. During the 50% MVT sustained task the increase in MUDR occurred earlier, after ∼70-80% CT. Our results suggest that adjustments in MUDR during submaximal isometric contractions to failure are contraction modality- and intensity-dependent. KEY POINTS: During prolonged muscle contractions a constant motor output can be maintained by recruitment of additional motor units and adjustments in their discharge rate. Whilst contraction-induced decrements in neuromuscular function are known to depend on task demands, task-specificity of motor unit discharge behaviour adjustments is still unclear. In this study, we tracked and compared discharge activity of several concurrently active motor units in the vastii muscles during different submaximal isometric knee extension tasks to failure, including intermittent vs. sustained contraction modalities performed in the same intensity domain (Experiment 1), and two sustained contractions performed at different intensities (Experiment 2). During each task, motor units modulated their discharge rate in a distinct, biphasic manner, with the modulation pattern depending on contraction intensity and modality. These results provide insight into motoneuronal adjustments during contraction tasks posing different demands on the neuromuscular system.

Motor learning changes the axon initial segment of the spinal motoneuron.

We are studying the mechanisms of H-reflex operant conditioning, a simple form of learning. Modelling studies in the literature and our previous data suggested that changes in the axon initial segment (AIS) might contribute. To explore this, we used blinded quantitative histological and immunohistochemical methods to study in adult rats the impact of H-reflex conditioning on the AIS of the spinal motoneuron that produces the reflex. Successful, but not unsuccessful, H-reflex up-conditioning was associated with greater AIS length and distance from soma; greater length correlated with greater H-reflex increase. Modelling studies in the literature suggest that these increases may increase motoneuron excitability, supporting the hypothesis that they may contribute to H-reflex increase. Up-conditioning did not affect AIS ankyrin G (AnkG) immunoreactivity (IR), p-p38 protein kinase IR, or GABAergic terminals. Successful, but not unsuccessful, H-reflex down-conditioning was associated with more GABAergic terminals on the AIS, weaker AnkG-IR, and stronger p-p38-IR. More GABAergic terminals and weaker AnkG-IR correlated with greater H-reflex decrease. These changes might potentially contribute to the positive shift in motoneuron firing threshold underlying H-reflex decrease; they are consistent with modelling suggesting that sodium channel change may be responsible. H-reflex down-conditioning did not affect AIS dimensions. This evidence that AIS plasticity is associated with and might contribute to H-reflex conditioning adds to evidence that motor learning involves both spinal and brain plasticity, and both neuronal and synaptic plasticity. AIS properties of spinal motoneurons are likely to reflect the combined influence of all the motor skills that share these motoneurons. KEY POINTS: Neuronal action potentials normally begin in the axon initial segment (AIS). AIS plasticity affects neuronal excitability in development and disease. Whether it does so in learning is unknown. Operant conditioning of a spinal reflex, a simple learning model, changes the rat spinal motoneuron AIS. Successful, but not unsuccessful, H-reflex up-conditioning is associated with greater AIS length and distance from soma. Successful, but not unsuccessful, down-conditioning is associated with more AIS GABAergic terminals, less ankyrin G, and more p-p38 protein kinase. The associations between AIS plasticity and successful H-reflex conditioning are consistent with those between AIS plasticity and functional changes in development and disease, and with those predicted by modelling studies in the literature. Motor learning changes neurons and synapses in spinal cord and brain. Because spinal motoneurons are the final common pathway for behaviour, their AIS properties probably reflect the combined impact of all the behaviours that use these motoneurons.

Disease Mechanisms and Therapeutic Approaches in SMARD1-Insights from Animal Models and Cell Models.

Spinal muscular atrophy with respiratory distress type 1 (SMARD1) is a fatal childhood motoneuron disease caused by mutations in the IGHMBP2 gene. It is characterized by muscle weakness, initially affecting the distal extremities due to the degeneration of spinal α-motoneurons, and respiratory distress, due to the paralysis of the diaphragm. Infantile forms with a severe course of the disease can be distinguished from juvenile forms with a milder course. Mutations in the IGHMBP2 gene have also been found in patients with peripheral neuropathy Charcot-Marie-Tooth type 2S (CMT2S). IGHMBP2 is an ATP-dependent 5'→3' RNA helicase thought to be involved in translational mechanisms. In recent years, several animal models representing both SMARD1 forms and CMT2S have been generated to initially study disease mechanisms. Later, the models showed very well that both stem cell therapies and the delivery of the human IGHMBP2 cDNA by AAV9 approaches (AAV9-IGHMBP2) can lead to significant improvements in disease symptoms. Therefore, the SMARD1 animal models, in addition to the cellular models, provide an inexhaustible source for obtaining knowledge of disease mechanisms, disease progression at the cellular level, and deeper insights into the development of therapies against SMARD1.

Roxithromycin exposure induces motoneuron malformation and behavioral deficits of zebrafish by interfering with the differentiation of motor neuron progenitor cells.

Roxithromycin (ROX), a commonly used macrolide antibiotic, is extensively employed in human medicine and livestock industries. Due to its structural stability and resistance to biological degradation, ROX persists as a resilient environmental contaminant, detectable in aquatic ecosystems and food products. However, our understanding of the potential health risks to humans from continuous ROX exposure remains limited. In this study, we used the zebrafish as a vertebrate model to explore the potential developmental toxicity of early ROX exposure, particularly focusing on its effects on locomotor functionality and CaP motoneuron development. Early exposure to ROX induces marked developmental toxicity in zebrafish embryos, significantly reducing hatching rates (n=100), body lengths (n=100), and increased malformation rates (n=100). The zebrafish embryos treated with a corresponding volume of DMSO (0.1%, v/v) served as vehicle controls (veh). Moreover, ROX exposure adversely affected the locomotive capacity of zebrafish embryos, and observations in transgenic zebrafish Tg(hb9:eGFP) revealed axonal loss in motor neurons, evident through reduced or irregular axonal lengths (n=80). Concurrently, abnormal apoptosis in ROX-exposed zebrafish embryos intensified alongside the upregulation of apoptosis-related genes (bax, bcl2, caspase-3a). Single-cell sequencing further disclosed substantial effects of ROX on genes involved in the differentiation of motor neuron progenitor cells (ngn1, olig2), axon development (cd82a, mbpa, plp1b, sema5a), and neuroimmunity (aplnrb, aplnra) in zebrafish larvae (n=30). Furthermore, the CaP motor neuron defects and behavioral deficits induced by ROX can be rescued by administering ngn1 agonist (n=80). In summary, ROX exposure leads to early-life abnormalities in zebrafish motor neurons and locomotor behavior by hindering the differentiation of motor neuron progenitor cells and inducing abnormal apoptosis.

NeuroMechanics: Electrophysiological and computational methods to accurately estimate the neural drive to muscles in humans in vivo.

The ultimate neural signal for muscle control is the neural drive sent from the spinal cord to muscles. This neural signal comprises the ensemble of action potentials discharged by the active spinal motoneurons, which is transmitted to the innervated muscle fibres to generate forces. Accurately estimating the neural drive to muscles in humans in vivo is challenging since it requires the identification of the activity of a sample of motor units (MUs) that is representative of the active MU population. Current electrophysiological recordings usually fail in this task by identifying small MU samples with over-representation of higher-threshold with respect to lower-threshold MUs. Here, we describe recent advances in electrophysiological methods that allow the identification of more representative samples of greater numbers of MUs than previously possible. This is obtained with large and very dense arrays of electromyographic electrodes. Moreover, recently developed computational methods of data augmentation further extend experimental MU samples to infer the activity of the full MU pool. In conclusion, the combination of new electrode technologies and computational modelling allows for an accurate estimate of the neural drive to muscles and opens new perspectives in the study of the neural control of movement and in neural interfacing.

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.

Modulation of central synapse remodeling after remote peripheral injuries by the CCL2-CCR2 axis and microglia.

Microglia-mediated synaptic plasticity after CNS injury varies depending on injury severity, but the mechanisms that adjust synaptic plasticity according to injury differences are largely unknown. This study investigates differential actions of microglia on essential spinal motor synaptic circuits following different kinds of nerve injuries. Following nerve transection, microglia and C-C chemokine receptor type 2 signaling permanently remove Ia axons and synapses from the ventral horn, degrading proprioceptive feedback during motor actions and abolishing stretch reflexes. However, Ia synapses and reflexes recover after milder injuries (nerve crush). These different outcomes are related to the length of microglia activation, being longer after nerve cuts, with slower motor-axon regeneration and extended expression of colony-stimulating factor type 1 in injured motoneurons. Prolonged microglia activation induces CCL2 expression, and Ia synapses recover after ccl2 is deleted from microglia. Thus, microglia Ia synapse removal requires the induction of specific microglia phenotypes modulated by nerve regeneration efficiencies. However, synapse preservation was not sufficient to restore the stretch-reflex function.

Sensory Stimulation of the Triceps Surae Muscle Complex Modulates Spinal Reflex Responses-A Comparison between Tapotement Massage and Repetitive Peripheral Magnetic Stimulation (rPMS).

BACKGROUND: The reduction of muscular hypertonia is important in the treatment of various diseases or rehabilitation. This study aims to test the efficacy of a 5 Hz mechanical muscle stimulation (tapotement massage) in comparison to a 5 Hz repetitive peripheral magnetic stimulation (rPMS) on the neuromuscular reflex response. METHODS: In a randomized control trial, 15 healthy volunteers were administered with either 5 Hz rPMS, tapotement massage, or rPMS sham stimulation. The posterior tibial nerve was stimulated with rPMS and sham stimulation. The Achilles tendon was exposed to a mechanically applied high-amplitude 5 Hz repetitive tendon tapotement massage (rTTM). The tendon reflex (TR) was measured for the spinal response of the soleus muscle. RESULTS: After rPMS, there was a reduction of the TR response (-9.8%, p ≤ 0.034) with no significant changes after sham stimulation. Likewise, TR decreased significantly (-17.4%, p ≤ 0.002) after Achilles tendon tapotement intervention. CONCLUSIONS: These findings support the hypothesis that both afferent 5 Hz sensory stimulations contributed to a modulation within the spinal and/or supraspinal circuits, which resulted in a reduction of the spinal reflex excitability. The effects could be beneficial for patients with muscle hypertonia and could improve the functional results of rehabilitation programs.

DLK signaling in axotomized neurons triggers complement activation and loss of upstream synapses.

Axotomized spinal motoneurons (MNs) lose presynaptic inputs following peripheral nerve injury; however, the cellular mechanisms that lead to this form of synapse loss are currently unknown. Here, we delineate a critical role for neuronal kinase dual leucine zipper kinase (DLK)/MAP3K12, which becomes activated in axotomized neurons. Studies with conditional knockout mice indicate that DLK signaling activation in injured MNs triggers the induction of phagocytic microglia and synapse loss. Aspects of the DLK-regulated response include expression of C1q first from the axotomized MN and then later in surrounding microglia, which subsequently phagocytose presynaptic components of upstream synapses. Pharmacological ablation of microglia inhibits the loss of cholinergic C boutons from axotomized MNs. Together, the observations implicate a neuronal mechanism, governed by the DLK, in the induction of inflammation and the removal of synapses.

hnRNP R regulates mitochondrial movement and membrane potential in axons of motoneurons.

Axonal mitochondria defects are early events in the pathogenesis of motoneuron disorders such as spinal muscular atrophy and amyotrophic lateral sclerosis. The RNA-binding protein hnRNP R interacts with different motoneuron disease-related proteins such as SMN and TDP-43 and has important roles in axons of motoneurons, including axonal mRNA transport. However, whether hnRNP R also modulates axonal mitochondria is currently unknown. Here, we show that axonal mitochondria exhibit altered function and motility in hnRNP R-deficient motoneurons. Motoneurons lacking hnRNP R show decreased anterograde and increased retrograde transport of mitochondria in axons. Furthermore, hnRNP R-deficiency leads to mitochondrial hyperpolarization, caused by decreased complex I and reversed complex V activity within the respiratory chain. Taken together, our data indicate a role for hnRNP R in regulating transport and maintaining functionality of axonal mitochondria in motoneurons.

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.

Transient Synaptic Enhancement Triggered by Exogenously Supplied Monocarboxylate in Drosophila Motoneuron Synapse.

Current evidence suggests that glial cells provide C3 carbon sources to fuel neuronal activity; however, this notion has become challenged by biosensor studies carried out in acute brain slices or in vivo, showing that neuronal activity does not rely on the import of astrocyte-produced L-lactate. Rather, stimulated neurons become net lactate exporters, as it was also shown in Drosophila neurons, in which astrocyte-provided lactate returns as lipid droplets to be stored in glial cells. In this view, we investigate whether exogenously supplied monocarboxylates can support Drosophila motoneuron neurotransmitter release (NTR). By assessing the excitatory post-synaptic current (EPSC) amplitude under voltage-clamp as NTR indicative, we found that both pyruvate and L-lactate, as the only carbon sources in the synapses bathing-solution, cause a large transient NTR enhancement, which declines to reach a synaptic depression state, from which the synapses do not recover. The FM1-43 pre-synaptic loading ability, however, is maintained under monocarboxylate, suggesting that SV cycling should not contribute to the synaptic depression state. The NTR recovery was reached by supplementing the monocarboxylate medium with sucrose. However, monocarboxylate addition to sucrose medium does not enhance NTR, but it does when the disaccharide concentration becomes too reduced. Thus, when pyruvate concentrations become too reduced, exogenously supplied L-lactate could be converted to pyruvate and metabolized by the neural mitochondria, triggering the NTR enhancement. SIGNIFICANCE STATEMENT: The question of whether monocarboxylic acids can fuel the Drosophila motoneuron NTR was challenged. Our findings show that exogenously supplied monocarboxylates trigger a large transient synaptic enhancement just under extreme glycolysis reduction but fail to maintain NTR under sustained synaptic demand, still at low frequency stimulation, driven to the synapses to a synaptic depression state. Glycolysis activation, by adding sucrose to the monocarboxylate bath solution, restores the motoneuron NTR ability, giving place to a hexoses role in SV recruitment. Moreover these results suggest exogenously supplied C3 carbon sources could have an additional role beyond providing energetic support for neural activity.

The SMA Modifier Plastin 3 Targets Cell Membrane-Associated Proteins in Motoneurons.

Loss of the Survival Motor Neuron (SMN) gene inevitably leads to spinal muscular atrophy (SMA), one of the most common fatal neuromuscular diseases in children with FDA and EMA approved therapies. However, the cellular mechanisms leading to neuromuscular junction (NMJ) dysfunction due to impaired Ca2+ homeostasis in the presynaptic compartment remain largely unexplained. In the last decade, the so-called SMA modifiers have gained attention. The F-actin bundler Plastin 3 (PLS3) is one of them and counteracts neurotransmission defects, including altered vesicle endocytosis, in Smn-deficient NMJs. Properly bundled F-actin is the basis for the translocation and arrangement of transmembrane proteins at the cell surface. Our recently published data by Hennlein et al., J Cell Biol. (2023) clearly showed that Smn deficiency impairs the F-actin dependent translocation of the high-affinity BDNF receptor TrkB to the cell surface resulting in reduced BDNF-mediated TrkB activation in motor axon terminals. Strikingly, the overexpression of PLS3 restores TrkB availability, and significantly improves the clustering of the active zone-associated voltage-gated calcium channel Cav2.2 in growth cones of Smn-deficient motoneurons. These observations raise the question of how PLS3 mediates the proper cell surface localization and cluster-like formation of Cav2.2 in motor axon terminals.

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.

ICA-27243 improves neuromuscular function and preserves motoneurons in the transgenic SOD1G93A mice.

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by the death of upper and lower motor neurons (MNs). Excessive neuronal excitability has been implicated in MN degeneration; thus, modulation of hyperexcitability appears as a promising therapeutic strategy. Potassium channels are attractive targets since they can be activated at subthreshold voltages and can regulate neuronal excitability. In this study, we assayed the effects of N-(6-Chloro-pyridin-3-yl)-3,4-difluorobenzamide compound, known as ICA-27243, as a potential treatment for ALS. ICA-27243 is a highly selective Kv7.2/7.3 opener used mainly in epilepsy models. In the in vitro model of spinal cord organotypic cultures (SCOCs) exposed to acute excitotoxicity, ICA-27243 prevented MN degeneration at a dose-of 10 â€‹µM. Administration of ICA-27243 to transgenic SOD1G93A ALS mice improved the decline of neuromuscular function, maintained locomotion and coordination in the rotarod, decreased spinal MN death and attenuated glial reactivity. In conclusion, we report here for the first time that ICA-27243 is an effective treatment for ALS, emphasizing the potential of targeting Kv channels to reduce neuronal hyperexcitability.

Knockdown of calpain1 in lumbar motoneurons reduces spasticity after spinal cord injury in adult rats.

Spasticity, affecting ∼75% of patients with spinal cord injury (SCI), leads to hyperreflexia, muscle spasms, and cocontractions of antagonist muscles, greatly affecting their quality of life. Spasticity primarily stems from the hyperexcitability of motoneurons below the lesion, driven by an upregulation of the persistent sodium current and a downregulation of chloride extrusion. This imbalance results from the post-SCI activation of calpain1, which cleaves Nav1.6 channels and KCC2 cotransporters. Our study was focused on mitigating spasticity by specifically targeting calpain1 in spinal motoneurons. We successfully transduced lumbar motoneurons in adult rats with SCI using intrathecal administration of adeno-associated virus vector serotype 6, carrying a shRNA sequence against calpain1. This approach significantly reduced calpain1 expression in transduced motoneurons, leading to a noticeable decrease in spasticity symptoms, including hyperreflexia, muscle spasms, and cocontractions in hindlimb muscles, which are particularly evident in the second month post-SCI. In addition, this decrease, which prevented the escalation of spasticity to a severe grade, paralleled the restoration of KCC2 levels in transduced motoneurons, suggesting a reduced proteolytic activity of calpain1. These findings demonstrate that inhibiting calpain1 in motoneurons is a promising strategy for alleviating spasticity in SCI patients.