In Vivo Electrophysiological Measurement of Compound Muscle Action Potential from the Forelimbs in Mouse Models of Motor Neuron Degeneration

Advancements in stimulation therapies for peripheral nerve regeneration

Soft-tissue injuries affecting muscles, nerves, vasculature, tendons, and ligaments often diminish the quality of life due to pain, loss of function, and financial burdens. Both natural healing and surgical interventions can result in scarring, which potentially may impede functional recovery and lead to persistent pain. Scar tissue, characterized by a highly disorganized fibrotic extracellular matrix, may serve as a physical barrier to regeneration and drug delivery. While approaches such as drugs, biomaterials, cells, external stimulation, and other physical forces show promise in mitigating scarring and promoting regenerative healing, their implementation remains limited and challenging. Ultrasound, laser, electrical, and magnetic forms of external stimulation have been utilized to promote soft tissue as well as neural tissue regeneration. After stimulation, neural tissues experience increased proliferation of Schwann cells, secretion of neurotropic factors, production of myelin, and growth of vasculature, all aimed at supporting axon regeneration and innervation. Yet, the outcomes of healing vary depending on the pathophysiology of the damaged nerve, the timing of stimulation following injury, and the specific parameters of stimulation employed. Increased treatment intensity and duration have been noted to hinder the healing process by inducing tissue damage. These stimulation modalities, either alone or in combination with nerve guidance conduits and scaffolds, have been demonstrated to promote healing. However, the literature currently lacks a detailed understanding of the stimulation parameters used for nerve healing applications. In this article, we aim to address this gap by summarizing existing reports and providing an overview of stimulation parameters alongside their associated healing outcomes.

Model-based interspecies interpretation of botulinum neurotoxin type A on muscle-contraction inhibition

Botulinum neurotoxins (BoNTs) are commonly used in therapeutic and cosmetic applications. One such neurotoxin, BoNT type A (BoNT/A), has been studied widely for its effects on muscle function and contraction. Despite the importance of BoNT/A products, determining the blood concentrations of these toxins can be challenging. To address this, researchers have focused on pharmacodynamic (PD) markers, including compound muscle action potential (CMAP) and digit abduction scoring (DAS). In this study, we aimed to develop a probabilistic kinetic-pharmacodynamic (K-PD) model to interpret CMAP and DAS data obtained from mice and rats during the development of BoNT/A products. The researchers also wanted to gain a better understanding of how the estimated parameters from the model relate to the bridging of animal models to human responses. We used female Institute of Cancer Research mice and Sprague-Dawley (SD) rats to measure CMAP and DAS levels over 32 weeks after administering BoNT/A. We developed a muscle-contraction inhibition model using a virtual pharmacokinetic (PK) compartment combined with an indirect response model and performed model diagnostics using goodness-of-fit analysis, visual predictive checks (VPC), and bootstrap analysis. The CMAP and DAS profiles were dose-dependent, with recovery times varying depending on the administered dose. The final K-PD model effectively characterized the data and provided insights into species-specific differences in the PK and PD parameters. Overall, this study demonstrated the utility of PK-PD modeling in understanding the effects of BoNT/A and provides a foundation for future research on other BoNT/A products.

The analysis of the skeletal muscle metabolism is crucial for designing optimal exercise paradigms in type 2 diabetes mellitus

BACKGROUND

Type 2 diabetes mellitus (T2DM) is a chronic metabolic disease that commonly results from a high-calorie diet and sedentary lifestyle, leading to insulin resistance and glucose homeostasis perturbation. Physical activity is recommended as one first-line treatment in T2DM, but it leads to contrasted results. We hypothesized that, instead of applying standard exercise protocols, the prescription of personalized exercise programs specifically designed to reverse the potential metabolic alterations in skeletal muscle could result in better results.

METHODS

To test this hypothesis, we drew the metabolic signature of the fast-twitch quadriceps muscle, based on a combined unbiased NMR spectroscopy and RT-qPCR study, in several T2DM mouse models of different genetic background (129S1/SvImJ, C57Bl/6J), sex and aetiology (high-fat diet (HFD) or HFD/Streptozotocin (STZ) induction or transgenic MKR (FVB-Tg Ckm-IGF1R*K1003R)1Dlr/J) mice. Three selected mouse models with unique muscular metabolic signatures were submitted to three different swimming-based programs, designed to address each metabolic specificity.

RESULTS

We found that depending on the genetic background, the sex, and the mode of T2DM induction, specific muscular adaptations occurred, including depressed glycolysis associated with elevated PDK4 expression, shift to β-oxidation, or deregulation of amino-acid homeostasis. Interestingly, dedicated swimming-based exercises designed to restore specific metabolic alterations in muscle were found optimal in improving systemic T2DM hallmarks, including a significant reduction in insulin resistance, the improvement of glucose homeostasis, and a delay in sensorimotor function alterations.

CONCLUSION

The muscle metabolism constitutes an important clue for the design of precision exercises with potential clinical implications for T2DM patients.

A pathologically expanded, clonal lineage of IL-21 producing CD4+ T cells drives Inflammatory neuropathy

Inflammatory neuropathies, which include CIDP (chronic inflammatory demyelinating polyneuropathy) and GBS (Guillain Barre Syndrome), result from autoimmune destruction of the peripheral nervous system (PNS) and are characterized by progressive weakness and sensory loss. CD4+ T cells play a key role in the autoimmune destruction of the PNS. Yet, key properties of pathogenic CD4+ T cells remain incompletely understood. Here, we use paired scRNAseq and scTCRseq of peripheral nerves from an inflammatory neuropathy mouse model to identify IL-21 expressing CD4+ T cells that are clonally expanded and multifunctional. These IL-21-expressing CD4+ T cells are comprised of two transcriptionally distinct expanded populations, which express genes associated with Tfh and Tph subsets. Remarkably, TCR clonotypes are shared between these two IL-21-expressing populations, suggesting a common lineage differentiation pathway. Finally, we demonstrate that IL-21 signaling is required for neuropathy development and pathogenic T cell infiltration into peripheral nerves. IL-21 signaling upregulates CXCR6, a chemokine receptor that promotes CD4+ T cell localization in peripheral nerves. Together, these findings point to IL-21 signaling, Tfh/Tph differentiation, and CXCR6-mediated cellular localization as potential therapeutic targets in inflammatory neuropathies.

Mutation in the FUS nuclear localisation signal domain causes neurodevelopmental and systemic metabolic alterations

Variants in the ubiquitously expressed DNA/RNA-binding protein FUS cause aggressive juvenile forms of amyotrophic lateral sclerosis (ALS). Most FUS mutation studies have focused on motor neuron degeneration; little is known about wider systemic or developmental effects. We studied pleiotropic phenotypes in a physiological knock-in mouse model carrying the pathogenic FUSDelta14 mutation in homozygosity. RNA sequencing of multiple organs aimed to identify pathways altered by the mutant protein in the systemic transcriptome, including metabolic tissues, given the link between ALS-frontotemporal dementia and altered metabolism. Few genes were commonly altered across all tissues, and most genes and pathways affected were generally tissue specific. Phenotypic assessment of mice revealed systemic metabolic alterations related to the pathway changes identified. Magnetic resonance imaging brain scans and histological characterisation revealed that homozygous FUSDelta14 brains were smaller than heterozygous and wild-type brains and displayed significant morphological alterations, including a thinner cortex, reduced neuronal number and increased gliosis, which correlated with early cognitive impairment and fatal seizures. These findings show that the disease aetiology of FUS variants can include both neurodevelopmental and systemic alterations.

[Decreased Expression of Mitochondrial Calcium Uptake Protein 1 Leads to Skeletal Muscle Dysfunction in Septic Mice]

Objective

To observe the effect of sepsis on skeletal muscle function and to explore the role of skeletal muscle mitochondrial calcium uptake protein 1 (MICU1).

Methods

A total of 40 specific-pathogen-free (SPF) healthy male C57BL/6J mice were randomly assigned to 4 groups, a sham operation group (Sham group, n=8), a sepsis modeling 6 h group (cecal ligation and puncture [CLP]-6 h group, n=10), a sepsis modeling 12 h group (CLP-12 h group, n=10), and a sepsis modeling 24 h group (CLP-24 h, n=12). The sepsis model was established by CLP. Mice in the Sham group only underwent laparotomic exploration of the cecum. Another 20 SPF mice were selected. The tibialis anterior muscle on one side was empty-transfected with adeno-associated virus (AAV) as controls (AAV-C), and the tibialis anterior muscle on the other side was transfected with AAV to enhance MICU1 expression (AAV-M). The mice were randomly assigned to two groups, a sham operation group (AAV-C-Sham and AAV-M-Sham, n=8) and a sepsis model 24 h group (AAV-C-CLP and AAV-M-CLP, n=12). The grip strength and compound muscle action potential (CMAP) of the tibialis anterior muscle were measured in each group at the corresponding time points. The levels of inflammatory factors, including tumor necrosis factor α (TNF-α) and interleukin 6 (IL-6), in the skeletal muscle were measured by ELISA. The morphological changes of skeletal muscle cells were observed through H&E staining. The expression levels of MICU1 and muscle atrophy-related proteins, including muscle RING-finger containing protein 1 (MuRF1) and muscle atrophy Fbox protein (MAFbx), were determined by Western blot. The expression levels of MICU1 mRNA in skeletal muscle were determined by RT-qPCR.

Results

Compared with mice in the Sham group, mice in the CLP group showed decreased body weight ( P<0.05); their grip strength decreased with the prolongation of CLP modeling time ( P<0.05); the amplitude of CMAP decreased, showing prolonged duration and latency ( P<0.05); the expression levels of inflammatory factors, including TNF-α and IL-6, in skeletal muscle increased gradually ( P<0.05); the fiber diameter and cross-sectional area of skeletal muscle decreased gradually with the prolongation of modeling time ( P<0.05); the protein expression levels of MuRF1and MAFbx proteins increased gradually ( P<0.05); the expression levels of MICU1 protein and mRNA decreased gradually ( P<0.05). There was no significant difference in all indices between AAV-M-Sham and AAV-C-Sham groups ( P>0.05). Compared with mice in the AAV-C-CLP group, mice in the AAV-M-CLP group showed increased grip strength ( P<0.05); the amplitude of CMAP increased, showing shortened duration and latency ( P <0.05); the fiber diameter and cross-sectional area of skeletal muscle increased ( P<0.05); the expression levels of MuRF1and MAFbx decreased ( P<0.05).

Conclusion

Sepsis leads to skeletal muscle dysfunction, which is related to the decrease in mitochondrial MICU1 expression.

In vivo imaging of axonal transport in peripheral nerves of rodent forelimbs

Axonal transport is the essential process by which neurons actively traffic a variety of cargoes between the cell soma and axon terminals. Accordingly, dysfunctional axonal transport is linked to many nervous system conditions. Therefore, being able to image and quantify this dynamic process in live neurons of animal disease models is beneficial for understanding neuropathology and testing new therapies at the preclinical level. As such, intravital approaches have been developed to assess cargo movement in the hindlimb sciatic nerves of live, anaesthetised mice. Here, we describe an adapted method for in vivo imaging of axonal transport in intact median and ulnar nerves of the rodent forelimb. Injection of a fluorescently labelled and non-toxic fragment of tetanus neurotoxin (H_CT) into the mouse forepaw permits the identification of signalling endosomes in intact axons of median and ulnar nerves. Through immunofluorescent analysis of forelimb lumbrical muscles and median/ulnar nerves, we confirmed that H_CT is taken up at motor nerve terminals and predominantly locates to motor axons. We then showed that the baseline trafficking of signalling endosomes is similar between the median/ulnar nerves and the sciatic nerve in adult wild-type mice. Importantly, this adapted method can be readily tailored for assessment of additional cargoes, such as mitochondria. By measuring transport in forelimb and hindlimb nerves, comparative anatomical and functional analyses can be performed in rodent disease models to aid our understanding of peripheral nerve disease pathogenesis and response to injury.

HDAC3 Inhibition Stimulates Myelination in a CMT1A Mouse Model

Charcot-Marie-Tooth disease (CMT) is the most common inherited peripheral neuropathy, with currently no effective treatment or cure. CMT1A is caused by a duplication of the PMP22 gene, which leads to Schwann cell differentiation defects and dysmyelination of the peripheral nerves. The epigenetic regulator histone deacetylase 3 (HDAC3) has been shown to negatively regulate myelination as well as its associated signaling pathways, PI3K-AKT and MAPK-ERK. We showed that these signaling pathways are indeed downregulated in the C3-PMP22 mouse model, similar to what has been shown in the CMT1A rat model. We confirmed that early postnatal defects are present in the peripheral nerves of the C3-PMP22 mouse model, which led to a progressive reduction in axon caliber size and myelination. The aim of this study was to investigate whether pharmacological HDAC3 inhibition could be a valuable therapeutic approach for this CMT1A mouse model. We demonstrated that early treatment of CMT1A mice with the selective HDAC3 inhibitor RGFP966 increased myelination and myelin g-ratios, which was associated with improved electrophysiological recordings. However, a high dose of RGFP966 caused a decline in rotarod performance and a decline in overall grip strength. Additionally, macrophage presence in peripheral nerves was increased in RGFP966 treated CMT1A mice. We conclude that HDAC3 does not only play a role in regulating myelination but is also important in the neuroimmune modulation. Overall, our results indicate that correct dosing of HDAC3 inhibitors is of crucial importance if translated to a clinical setting for demyelinating forms of CMT or other neurological disorders.

Neurotransmission Recovery by Melatonin Measured by CMAP

Compound muscle action potential (CMAP) recordings provide a sensitive electromyographic approach to measure nerve conduction and assess neuromuscular junction functionality in humans and rodents. In humans, it represents a diagnostic tool for neuromuscular disorders. In rodents, this approach is widely employed to dissect the molecular mechanisms driving peripheral nerve degeneration/regeneration, as well as to evaluate the effect of candidate pro-regenerative compounds. The method described here allows recording CMAP from the gastrocnemius muscle of mice after sciatic nerve stimulation. We report some representative traces of CMAP recorded from adult, healthy mice, after sciatic nerve compression and during neurotransmission recovery stimulated by melatonin administration.

Ankyrin-dependent Na+ channel clustering prevents neuromuscular synapse fatigue

Skeletal muscle contraction depends on activation of clustered acetylcholine receptors (AchRs) and muscle-specific Na⁺ channels (Nav1.4). Some Nav1.4 channels are highly enriched at the neuromuscular junction (NMJ), and their clustering is thought to be essential for effective muscle excitation. However, this has not been experimentally tested, and how NMJ Na⁺ channels are clustered is unknown. Here, using muscle-specific ankyrinR, ankyrinB, and ankyrinG single, double, and triple-conditional knockout mice, we show that Nav1.4 channels fail to cluster only after deletion of all three ankyrins. Remarkably, ankyrin-deficient muscles have normal NMJ morphology, AchR clustering, sarcolemmal levels of Nav1.4, and muscle force, and they show no indication of degeneration. However, mice lacking clustered NMJ Na⁺ channels have significantly reduced levels of motor activity and their NMJs rapidly fatigue after repeated nerve-dependent stimulation. Thus, the triple redundancy of ankyrins facilitates NMJ Na⁺ channel clustering to prevent neuromuscular synapse fatigue.