Morphological analysis of neuromuscular junction development and degeneration in rodent lumbrical muscles

Conductive MeCbl/PEDOT:PSS/HA hydrogels with electrical stimulation for enhanced peripheral nerve regeneration

Peripheral nerve regeneration (PNR) represents a substantial challenge in the medical field, primarily due to the limited regenerative capacity of the peripheral nerve system (PNS). Current research efforts are focused on developing advanced medical polymer materials to enhance nerve recovery. Despite significant progress, several critical issues remain unresolved, including biocompatibility, stability, mechanical strength, controlled degradation rates, and sustained release of therapeutic agents. This study examines the utilization of hyaluronic acid hydrogels, doped with mecobalamin (MeCbl) and conductive poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), in combination with exogenous electrical stimulation (ES) for PNR of rats. The strategy utilizes the MeCbl hydrogel to create a regenerative microenvironment and provide nutritional support for nerve cells, while PEDOT:PSS facilitates enhanced electrical signal conduction. ES has been shown to promote PNR and functional recovery, thereby demonstrating considerable potential. This study aims to comprehensively analyze the synergistic effects and potential value of this combined therapeutic approach, providing novel insights and pathways for the effective PNR.

Integrating postsynaptic morphology and dynamics to evaluate neuromuscular synapse status: Insights from α-bungarotoxin

The neuromuscular junction (NMJ) is a crucial peripheral synapse that controls muscle contraction. It consists of a presynaptic motor terminal, a postsynaptic muscle domain, and associated cells, such as terminal Schwann cells and kranocytes. Its larger size compared to central synapses has allowed detailed analyses of NMJ morphology that have been widely used as a reliable parameter of synaptic formation, maturation, function, and decline. Due to its high affinity for postsynaptic acetylcholine receptors (AChRs), the snake venom-derived α-bungarotoxin (BTX) has been pivotal in advancing our understanding of NMJ organization, enabling a detailed mapping of postsynaptic morphologies associated to distinct functional outcomes. Although certain morphological features are often associated with NMJ worsening, some of these cellular changes also occur in biological contexts where synaptic function remains intact. In this review, we draw on previous studies and our recent findings using BTX-based pulse-chase assays to suggest that combining morphological analyses with assessments of postsynaptic stability offers a more comprehensive understanding of NMJ function and regenerative potential. We propose that integrating diverse BTX-based tools into studies of NMJ morphology and stability will provide particularly valuable insights in contexts such as aging, injury, and neuromuscular diseases, where these combined parameters may serve as robust predictors of functional outcomes.

Non-muscle myosin II inhibition at the site of axon injury increases axon regeneration

Motor axon regeneration following peripheral nerve injury is critical for motor recovery but therapeutic interventions enhancing this are not available. We conduct a phenotypic screen on human motor neurons and identified blebbistatin, a non-muscle myosin II inhibitor, as the most effective neurite outgrowth promotor. Despite its efficacy in vitro, its poor bioavailability limits in vivo application. We, therefore, utilize a blebbistatin analog, NMIIi2, to explore its therapeutic potential for promoting axon regeneration. Local NMIIi2 application directly to injured axons enhances regeneration in human motor neurons. Furthermore, following a sciatic nerve crush injury in male mice, local NMIIi2 administration to the axonal injury site facilitates motor neuron regeneration, muscle reinnervation, and functional recovery. NMIIi2 also promotes axon regeneration in sensory, cortical, and retinal ganglion neurons. These findings highlight the therapeutic potential of topical NMII inhibition for treating axon damage.

Sex-specific astrocyte regulation of spinal motor circuits by Nkx6.1

Astrocytes exhibit diverse cellular and molecular properties across the central nervous system (CNS). Recent studies identified region-specific transcription factors (TF) that oversee these diverse properties; how sex differences intersect with region-specific transcriptional programs to regulate astrocyte function is unknown. Here, we show that the TF Nkx6.1 is specifically expressed in ventral astrocytes of the spinal cord and that its deletion results in sex-specific effects on astrocyte morphology. Astrocytes from males exhibit enhanced morphological complexity, accompanied by increased motor function and cholinergic synapses. In contrast, female astrocytes exhibit reduced complexity and no changes in motor function. Mechanistically, we found that Nkx6.1 exhibits sex-specific DNA-binding properties and epigenomic remodeling, identifying Semaphorin 4A (Sema4A) and Gabbr1 as targets regulating astrocyte morphology and cholinergic synapse formation. Collectively, our studies identify astrocytic Nkx6.1 as a key regulator of astrocyte properties in the spinal cord while adding sexual dimorphism as a layer of transcriptional regulation to astrocyte function and circuit activity.

Severe dynein dysfunction in cholinergic neurons exacerbates ALS-like phenotypes in a new mouse model

Cytoplasmic dynein 1, a motor protein essential for retrograde axonal transport, is increasingly implicated in the pathogenesis of neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). In this study, we developed a novel mouse model that combines the Legs at odd angles (Loa, F580Y) point mutation in the dynein heavy chain with a cholinergic neuron-specific knockout of the dynein heavy chain. This model, for the first time, allows us to investigate the impact of Loa allele exclusivity in these neurons into adulthood. Our findings reveal that this selective increase in dynein dysfunction exacerbated the phenotypes observed in heterozygous Loa mice including pre-wean survival, reduced body weight and grip strength. Additionally, it induced ALS-like pathology in neuromuscular junctions (NMJs) not seen in heterozygous Loa mice. Notably, we also found a previously unobserved significant increase in neurons displaying TDP-43 puncta in both Loa mutants, suggesting early TDP-43 mislocalisation - a hallmark of ALS. The novel model also exhibited a concurrent rise in p62 puncta that did not co-localise with TDP-43, indicating broader impairments in autophagic clearance mechanisms. Overall, this new model underscores the fact that dynein impairment alone can induce ALS-like pathology and provides a valuable platform to further explore the role of dynein in ALS.

Boosting BDNF in muscle rescues impaired axonal transport in a mouse model of DI-CMTC peripheral neuropathy

Charcot-Marie-Tooth disease (CMT) is a genetic peripheral neuropathy caused by mutations in many functionally diverse genes. The aminoacyl-tRNA synthetase (ARS) enzymes, which transfer amino acids to partner tRNAs for protein synthesis, represent the largest protein family genetically linked to CMT aetiology, suggesting pathomechanistic commonalities. Dominant intermediate CMT type C (DI-CMTC) is caused by YARS1 mutations driving a toxic gain-of-function in the encoded tyrosyl-tRNA synthetase (TyrRS), which is mediated by exposure of consensus neomorphic surfaces through conformational changes of the mutant protein. In this study, we first showed that human DI-CMTC-causing TyrRSE196K mis-interacts with the extracellular domain of the BDNF receptor TrkB, an aberrant association we have previously characterised for several mutant glycyl-tRNA synthetases linked to CMT type 2D (CMT2D). We then performed temporal neuromuscular assessments of YarsE196K mice modelling DI-CMT. We determined that YarsE196K homozygotes display a selective, age-dependent impairment in in vivo axonal transport of neurotrophin-containing signalling endosomes, phenocopying CMT2D mice. This impairment is replicated by injection of recombinant TyrRSE196K, but not TyrRSWT, into muscles of wild-type mice. Augmenting BDNF in DI-CMTC muscles, through injection of recombinant protein or muscle-specific gene therapy, resulted in complete axonal transport correction. Therefore, this work identifies a non-cell autonomous pathomechanism common to ARS-related neuropathies, and highlights the potential of boosting BDNF levels in muscles as a therapeutic strategy.

Disruption of Neuromuscular Junction Following Spinal Cord Injury and Motor Neuron Diseases

The neuromuscular junction (NMJ) is a crucial structure that connects the cholinergic motor neurons to the muscle fibers and allows for muscle contraction and movement. Despite the interruption of the supraspinal pathways that occurs in spinal cord injury (SCI), the NMJ, innervated by motor neurons below the injury site, has been found to remain intact. This highlights the importance of studying the NMJ in rodent models of various nervous system disorders, such as amyotrophic lateral sclerosis (ALS), Charcot-Marie-Tooth disease (CMT), spinal muscular atrophy (SMA), and spinal and bulbar muscular atrophy (SBMA). The NMJ is also involved in myasthenic disorders, such as myasthenia gravis (MG), and is vulnerable to neurotoxin damage. Thus, it is important to analyze the integrity of the NMJ in rodent models during the early stages of the disease, as this may allow for a better understanding of the condition and potential treatment options. The spinal cord also plays a crucial role in the functioning of the NMJ, as the junction relays information from the spinal cord to the muscle fibers, and the integrity of the NMJ could be disrupted by SCI. Therefore, it is vital to study SCI and muscle function when studying NMJ disorders. This review discusses the formation and function of the NMJ after SCI and potential interventions that may reverse or improve NMJ dysfunction, such as exercise, nutrition, and trophic factors.

Boosting BDNF in muscle rescues impaired axonal transport in a mouse model of DI-CMTC peripheral neuropathy

Charcot-Marie-Tooth disease (CMT) is a genetic peripheral neuropathy caused by mutations in many functionally diverse genes. The aminoacyl-tRNA synthetase (ARS) enzymes, which transfer amino acids to partner tRNAs for protein synthesis, represent the largest protein family genetically linked to CMT aetiology, suggesting pathomechanistic commonalities. Dominant intermediate CMT type C (DI-CMTC) is caused by YARS1 mutations driving a toxic gain-of-function in the encoded tyrosyl-tRNA synthetase (TyrRS), which is mediated by exposure of consensus neomorphic surfaces through conformational changes of the mutant protein. In this study, we first showed that human DI-CMTC-causing TyrRSE196K mis-interacts with the extracellular domain of the BDNF receptor TrkB, an aberrant association we have previously characterised for several mutant glycyl-tRNA synthetases linked to CMT type 2D (CMT2D). We then performed temporal neuromuscular assessments of YarsE196K mice modelling DI-CMT. We determined that YarsE196K homozygotes display a selective, age-dependent impairment in in vivo axonal transport of neurotrophin-containing signalling endosomes, phenocopying CMT2D mice. This impairment is replicated by injection of recombinant TyrRSE196K, but not TyrRSWT, into muscles of wild-type mice. Augmenting BDNF in DI-CMTC muscles, through injection of recombinant protein or muscle-specific gene therapy, resulted in complete axonal transport correction. Therefore, this work identifies a non-cell autonomous pathomechanism common to ARS-related neuropathies, and highlights the potential of boosting BDNF levels in muscles as a therapeutic strategy.

ENGRAILED-1 transcription factor has a paracrine neurotrophic activity on adult spinal α-motoneurons

Several homeoprotein transcription factors transfer between cells and regulate gene expression, protein translation, and chromatin organization in recipient cells. ENGRAILED-1 is one such homeoprotein expressed in spinal V1 interneurons that synapse on α-motoneurons. Neutralizing extracellular ENGRAILED-1 by expressing a secreted single-chain antibody blocks its capture by spinal motoneurons resulting in α-motoneuron loss and limb weakness. A similar but stronger phenotype is observed in the Engrailed-1 heterozygote mouse, confirming that ENGRAILED-1 exerts a paracrine neurotrophic activity on spinal cord α-motoneurons. Intrathecal injection of ENGRAILED-1 leads to its specific internalization by spinal motoneurons and has long-lasting protective effects against neurodegeneration and weakness. Midbrain dopaminergic neurons express Engrailed-1 and, similarly to spinal cord α-motoneurons, degenerate in the heterozygote. We identify genes expressed in spinal cord motoneurons whose expression changes in mouse Engrailed-1 heterozygote midbrain neurons. Among these, p62/SQSTM1 shows increased expression during aging in spinal cord motoneurons in the Engrailed-1 heterozygote and upon extracellular ENGRAILED-1 neutralization. We conclude that ENGRAILED-1 might regulate motoneuron aging and has non-cell-autonomous neurotrophic activity.

Kinesin-5 inhibition improves neural regeneration in experimental autoimmune neuritis

BACKGROUND

Autoimmune neuropathies can result in long-term disability and incomplete recovery, despite adequate first-line therapy. Kinesin-5 inhibition was shown to accelerate neurite outgrowth in different preclinical studies. Here, we evaluated the potential neuro-regenerative effects of the small molecule kinesin-5 inhibitor monastrol in a rodent model of acute autoimmune neuropathies, experimental autoimmune neuritis.

METHODS

Experimental autoimmune neuritis was induced in Lewis rats with the neurogenic P2-peptide. At the beginning of the recovery phase at day 18, the animals were treated with 1 mg/kg monastrol or sham and observed until day 30 post-immunisation. Electrophysiological and histological analysis for markers of inflammation and remyelination of the sciatic nerve were performed. Neuromuscular junctions of the tibialis anterior muscles were analysed for reinnervation. We further treated human induced pluripotent stem cells-derived secondary motor neurons with monastrol in different concentrations and performed a neurite outgrowth assay.

RESULTS

Treatment with monastrol enhanced functional and histological recovery in experimental autoimmune neuritis. Motor nerve conduction velocity at day 30 in the treated animals was comparable to pre-neuritis values. Monastrol-treated animals showed partially reinnervated or intact neuromuscular junctions. A significant and dose-dependent accelerated neurite outgrowth was observed after kinesin-5 inhibition as a possible mode of action.

CONCLUSION

Pharmacological kinesin-5 inhibition improves the functional outcome in experimental autoimmune neuritis through accelerated motor neurite outgrowth and histological recovery. This approach could be of interest to improve the outcome of autoimmune neuropathy patients.

Boosting peripheral BDNF rescues impaired in vivo axonal transport in CMT2D mice

Gain-of-function mutations in the housekeeping gene GARS1, which lead to the expression of toxic versions of glycyl-tRNA synthetase (GlyRS), cause the selective motor and sensory pathology characterizing Charcot-Marie-Tooth disease (CMT). Aberrant interactions between GlyRS mutants and different proteins, including neurotrophin receptor tropomyosin receptor kinase receptor B (TrkB), underlie CMT type 2D (CMT2D); however, our pathomechanistic understanding of this untreatable peripheral neuropathy remains incomplete. Through intravital imaging of the sciatic nerve, we show that CMT2D mice displayed early and persistent disturbances in axonal transport of neurotrophin-containing signaling endosomes in vivo. We discovered that brain-derived neurotrophic factor (BDNF)/TrkB impairments correlated with transport disruption and overall CMT2D neuropathology and that inhibition of this pathway at the nerve-muscle interface perturbed endosome transport in wild-type axons. Accordingly, supplementation of muscles with BDNF, but not other neurotrophins, completely restored physiological axonal transport in neuropathic mice. Together, these findings suggest that selectively targeting muscles with BDNF-boosting therapies could represent a viable therapeutic strategy for CMT2D.

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.

Axon Initial Segments Are Required for Efficient Motor Neuron Axon Regeneration and Functional Recovery of Synapses

The axon initial segment (AIS) generates action potentials and maintains neuronal polarity by regulating the differential trafficking and distribution of proteins, transport vesicles, and organelles. Injury and disease can disrupt the AIS, and the subsequent loss of clustered ion channels and polarity mechanisms may alter neuronal excitability and function. However, the impact of AIS disruption on axon regeneration after injury is unknown. We generated male and female mice with AIS-deficient multipolar motor neurons by deleting AnkyrinG, the master scaffolding protein required for AIS assembly and maintenance. We found that after nerve crush, neuromuscular junction reinnervation was significantly delayed in AIS-deficient motor neurons compared with control mice. In contrast, loss of AnkyrinG from pseudo-unipolar sensory neurons did not impair axon regeneration into the intraepidermal nerve fiber layer. Even after AIS-deficient motor neurons reinnervated the neuromuscular junction, they failed to functionally recover because of reduced synaptic vesicle protein 2 at presynaptic terminals. In addition, mRNA trafficking was disrupted in AIS-deficient axons. Our results show that, after nerve injury, an intact AIS is essential for efficient regeneration and functional recovery of axons in multipolar motor neurons. Our results also suggest that loss of polarity in AIS-deficient motor neurons impairs the delivery of axonal proteins, mRNAs, and other cargoes necessary for regeneration. Thus, therapeutic strategies for axon regeneration must consider preservation or reassembly of the AIS.SIGNIFICANCE STATEMENT Disruption of the axon initial segment is a common event after nervous system injury. For multipolar motor neurons, we show that axon initial segments are essential for axon regeneration and functional recovery after injury. Our results may help explain injuries where axon regeneration fails, and suggest strategies to promote more efficient axon regeneration.

A method to identify, dissect and stain equine neuromuscular junctions for morphological analysis

Morphological study of the neuromuscular junction (NMJ), a specialised peripheral synapse formed between a lower motor neuron and skeletal muscle fibre, has significantly contributed to the understanding of synaptic biology and neuromuscular disease pathogenesis. Rodent NMJs are readily accessible, and research into conditions such as amyotrophic lateral sclerosis (ALS), Charcot–Marie–Tooth disease (CMT), and spinal muscular atrophy (SMA) has relied heavily on experimental work in these small mammals. However, given that nerve length dependency is an important feature of many peripheral neuropathies, these rodent models have clear shortcomings; large animal models might be preferable, but their size presents novel anatomical challenges. Overcoming these constraints to study the NMJ morphology of large mammalian distal limb muscles is of prime importance to increase cross‐species translational neuromuscular research potential, particularly in the study of long motor units. In the past, NMJ phenotype analysis of large muscle bodies within the equine distal pelvic limb, such as the tibialis cranialis, or within muscles of high fibrous content, such as the soleus, has posed a distinct experimental hurdle. We optimised a technique for NMJ location and dissection from equine pelvic limb muscles. Using a quantification method validated in smaller species, we demonstrate their morphology and show that equine NMJs can be reliably dissected, stained and analysed. We reveal that the NMJs within the equine soleus have distinctly different morphologies when compared to the extensor digitorum longus and tibialis cranialis muscles. Overall, we demonstrate that equine distal pelvic limb muscles can be regionally dissected, with samples whole‐mounted and their innervation patterns visualised. These methods will allow the localisation and analysis of neuromuscular junctions within the muscle bodies of large mammals to identify neuroanatomical and neuropathological features.

CRABP1-CaMKII-Agrn regulates the maintenance of neuromuscular junction in spinal motor neuron

Cellular retinoic acid-binding protein 1 (CRABP1) binds retinoic acid (RA) specifically in the cytoplasm with unclear functions. CRABP1 is highly and specifically expressed in spinal motor neurons (MNs). Clinical and pre-clinical data reveal a potential link between CRABP1 and MN diseases, including the amyotrophic lateral sclerosis (ALS). We established a sequenced MN-muscle co-differentiation system to engineer an in vitro functional 3D NMJ model for molecular studies and demonstrated that CRABP1 in MNs contributes to NMJ formation and maintenance. Consistently, Crabp1 knockout (CKO) mice exhibited an adult-onset ALS-like phenotype with progressively deteriorated NMJs, characterized with behavioral, EchoMRI, electrophysiological, histological, and immunohistochemical studies at 2-20-months old. Mechanistically, CRABP1 suppresses CaMKII activation to regulate neural Agrn expression and downstream muscle LRP4-MuSK signaling, thereby maintaining NMJ. A proof-of-concept was provided by specific re-expression of CRABP1 to rescue Agrn expression and the phenotype. This study identifies CRABP1-CaMKII-Agrn signaling as a physiological pre-synaptic regulator in the NMJ. This study also highlights a potential protective role of CRABP1 in the progression of NMJ deficits in MN diseases.

Studies on the Manner of Collateral Regeneration From Nerve Stem to Motor Endplate

Despite recent evidence suggesting that nerve transfer techniques help improve clinical outcomes, the underlying manner by which collateral-regenerated nerve enters skeletal muscles to restore an organized pattern of the neuromuscular junction (NMJ) is unclear. To construct the animal models of collateral regeneration, the proximal peroneal nerve was fixed to the distal tibial nerve stump. Three months after surgery, the spatial distribution of motor endplates (MEPs) and corresponding in-muscle nerve branches in long flexor digitorum muscles were observed with tissue optical clearing combined with light-sheet microscopy in transgenic fluorescent mice. The results showed that the number of fibers in the proximal donor peroneal nerve was 415 ± 11, while regenerated nerve fibers in the distal tibial stump were 781 ± 43, which indicates a collateral regeneration ratio of 1.88. The spatial distribution of MEPs was restored to an organized pattern of the lamella, and the corresponding in-muscle nerve branches reverted to the normal manner such as after collateral regeneration. Beyond this, the numbers of MEPs dominated by the single distal nerve fiber were 25.58 ± 0.50 and 26.42 ± 0.94, respectively (n = 6, p > 0.05, collateral regeneration group vs. normal group). However, the numbers of distal-regenerated nerve fibers were less than those in normal control groups (781 ± 43 vs. 914 ± 55, n = 6, p < 0.05), and the number and perforations of MEPs were lower than those in normal control groups as such. In summary, this is the first study to show the manner of collateral regeneration of the peripheral nerve that the smaller proximal donor nerve can sprout more axonal buds to connect distal larger nerves and finally restore to an organized pattern of lamella dominated by corresponding in-muscle nerve branches.

Effects of delayed repair of peripheral nerve injury on the spatial distribution of motor endplates in target muscle

Motor endplates (MEPs) are important sites of information exchange between motor neurons and skeletal muscle, and are distributed in an organized pattern of lamellae in the muscle. Delayed repair of peripheral nerve injury typically results in unsatisfactory functional recovery because of MEP degeneration. In this study, the mouse tibial nerve was transected and repaired with a biodegradable chitin conduit, immediately following or 1 or 3 months after the injury. Fluorescent α-bungarotoxin was injected to label MEPs. Tissue optical clearing combined with light-sheet microscopy revealed that MEPs were distributed in an organized pattern of lamellae in skeletal muscle after delayed repair for 1 and 3 months. However, the total number of MEPs, the number of MEPs per lamellar cluster, and the maturation of single MEPs in gastrocnemius muscle gradually decreased with increasing denervation time. These findings suggest that delayed repair can restore the spatial distribution of MEPs, but it has an adverse effect on the homogeneity of MEPs in the lamellar clusters and the total number of MEPs in the target muscle. The study procedures were approved by the Animal Ethics Committee of the Peking University People's Hospital (approval No. 2019PHC015) on April 8, 2019.

A Functional Human-on-a-Chip Autoimmune Disease Model of Myasthenia Gravis for Development of Therapeutics

Myasthenia gravis (MG) is a chronic and progressive neuromuscular disease where autoantibodies target essential proteins such as the nicotinic acetylcholine receptor (nAChR) at the neuromuscular junction (NMJ) causing muscle fatigue and weakness. Autoantibodies directed against nAChRs are proposed to work by three main pathological mechanisms of receptor disruption: blocking, receptor internalization, and downregulation. Current in vivo models using experimental autoimmune animal models fail to recapitulate the disease pathology and are limited in clinical translatability due to disproportionate disease severity and high animal death rates. The development of a highly sensitive antibody assay that mimics human disease pathology is desirable for clinical advancement and therapeutic development. To address this lack of relevant models, an NMJ platform derived from human iPSC differentiated motoneurons and primary skeletal muscle was used to investigate the ability of an anti-nAChR antibody to induce clinically relevant MG pathology in the serum-free, spatially organized, functionally mature NMJ platform. Treatment of the NMJ model with the anti-nAChR antibody revealed decreasing NMJ stability as measured by the number of NMJs before and after the synchrony stimulation protocol. This decrease in NMJ stability was dose-dependent over a concentration range of 0.01-20 μg/mL. Immunocytochemical (ICC) analysis was used to distinguish between pathological mechanisms of antibody-mediated receptor disruption including blocking, receptor internalization and downregulation. Antibody treatment also activated the complement cascade as indicated by complement protein 3 deposition near the nAChRs. Additionally, complement cascade activation significantly altered other readouts of NMJ function including the NMJ fidelity parameter as measured by the number of muscle contractions missed in response to increasing motoneuron stimulation frequencies. This synchrony readout mimics the clinical phenotype of neurological blocking that results in failure of muscle contractions despite motoneuron stimulations. Taken together, these data indicate the establishment of a relevant disease model of MG that mimics reduction of functional nAChRs at the NMJ, decreased NMJ stability, complement activation and blocking of neuromuscular transmission. This system is the first functional human in vitro model of MG to be used to simulate three potential disease mechanisms as well as to establish a preclinical platform for evaluation of disease modifying treatments (etiology).

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.

Perturbed BMP signaling and denervation promote muscle wasting in cancer cachexia

Most patients with advanced solid cancers exhibit features of cachexia, a debilitating syndrome characterized by progressive loss of skeletal muscle mass and strength. Because the underlying mechanisms of this multifactorial syndrome are incompletely defined, effective therapeutics have yet to be developed. Here, we show that diminished bone morphogenetic protein (BMP) signaling is observed early in the onset of skeletal muscle wasting associated with cancer cachexia in mouse models and in patients with cancer. Cancer-mediated factors including Activin A and IL-6 trigger the expression of the BMP inhibitor Noggin in muscle, which blocks the actions of BMPs on muscle fibers and motor nerves, subsequently causing disruption of the neuromuscular junction (NMJ), denervation, and muscle wasting. Increasing BMP signaling in the muscles of tumor-bearing mice by gene delivery or pharmacological means can prevent muscle wasting and preserve measures of NMJ function. The data identify perturbed BMP signaling and denervation of muscle fibers as important pathogenic mechanisms of muscle wasting associated with tumor growth. Collectively, these findings present interventions that promote BMP-mediated signaling as an attractive strategy to counteract the loss of functional musculature in patients with cancer.

Optimized design of a hyperflexible sieve electrode to enhance neurovascular regeneration for a peripheral neural interface

One in 190 Americans is currently living with the loss of a limb resulted from injury, amputation, or neurodegenerative disease. Advanced neuroprosthetic devices combine peripheral neural interfaces with sophisticated prosthetics and hold great potential for the rehabilitation of impaired motor and sensory functions. While robotic prosthetics have advanced very rapidly, peripheral neural interfaces have long been limited by the capability of interfacing with the peripheral nervous system. In this work, we developed a hyperflexible regenerative sieve electrode to serve as a peripheral neural interface. We examined tissue neurovascular integration through this novel device. We demonstrated that we could enhance the neurovascular invasion through the device with directional growth factor delivery. Furthermore, we demonstrated that we could reduce the tissue reaction to the device often seen in peripheral neural interfaces. Finally, we show that we can create a stable tissue device interface in a long-term implantation that does not impede the normal regenerative processes of the nerve. Our study developed an optimal platform for the continued development of hyperflexible sieve electrode peripheral neural interfaces.

Creating stem cell-derived neuromuscular junctions in vitro

Recent development of novel therapies has improved mobility and quality of life for people suffering from inheritable neuromuscular disorders. Despite this progress, the majority of neuromuscular disorders are still incurable, in part due to a lack of predictive models of neuromuscular junction (NMJ) breakdown. Improvement of predictive models of a human NMJ would be transformative in terms of expanding our understanding of the mechanisms that underpin development, maintenance, and disease, and as a testbed with which to evaluate novel therapeutics. Induced pluripotent stem cells (iPSCs) are emerging as a clinically relevant and non‐invasive cell source to create human NMJs to study synaptic development and maturation, as well as disease modeling and drug discovery. This review will highlight the recent advances and remaining challenges to generating an NMJ capable of eliciting contraction of stem cell‐derived skeletal muscle in vitro. We explore the advantages and shortcomings of traditional NMJ culturing platforms, as well as the pioneering technologies and novel, biomimetic culturing systems currently in use to guide development and maturation of the neuromuscular synapse and extracellular microenvironment. Then, we will explore how this NMJ‐in‐a‐dish can be used to study normal assembly and function of the efferent portion of the neuromuscular arc, and how neuromuscular disease‐causing mutations disrupt structure, signaling, and function.

Dissection, in vivo imaging and analysis of the mouse epitrochleoanconeus muscle

Analysis of rodent muscles affords an opportunity to glean key insights into neuromuscular development and the detrimental impact of disease‐causing genetic mutations. Muscles of the distal leg, for instance the gastrocnemius and tibialis anterior, are commonly used in such studies with mice and rats. However, thin and flat muscles, which can be dissected, processed and imaged without major disruption to muscle fibres and nerve‐muscle contacts, are more suitable for accurate and detailed analyses of the peripheral motor nervous system. One such wholemount muscle is the predominantly fast twitch epitrochleoanconeus (ETA), which is located in the upper forelimb, innervated by the radial nerve, and contains relatively large and uniformly flat neuromuscular junctions (NMJs). To facilitate incorporation of the ETA into the experimental toolkit of the neuromuscular disease field, here, we describe a simple method for its rapid isolation (<5 min), supported by high‐resolution videos and step‐by‐step images. Furthermore, we outline how the ETA can be imaged in live, anaesthetised mice, to enable examination of dynamic cellular processes occurring at the NMJ and within intramuscular axons, including transport of organelles, such as mitochondria and signalling endosomes. Finally, we present reference data on wild‐type ETA fibre‐type composition in young adult, male C57BL6/J mice. Comparative neuroanatomical studies of different muscles in rodent models of disease can generate critical insights into pathogenesis and pathology; dissection of the wholemount ETA provides the possibility to diversify the repertoire of muscles analysed for this endeavour.

NMJ-Analyser identifies subtle early changes in mouse models of neuromuscular disease

The neuromuscular junction (NMJ) is the peripheral synapse formed between a motor neuron axon terminal and a muscle fibre. NMJs are thought to be the primary site of peripheral pathology in many neuromuscular diseases, but innervation/denervation status is often assessed qualitatively with poor systematic criteria across studies, and separately from 3D morphological structure. Here, we describe the development of ‘NMJ-Analyser’, to comprehensively screen the morphology of NMJs and their corresponding innervation status automatically. NMJ-Analyser generates 29 biologically relevant features to quantitatively define healthy and aberrant neuromuscular synapses and applies machine learning to diagnose NMJ degeneration. We validated this framework in longitudinal analyses of wildtype mice, as well as in four different neuromuscular disease models: three for amyotrophic lateral sclerosis (ALS) and one for peripheral neuropathy. We showed that structural changes at the NMJ initially occur in the nerve terminal of mutant TDP43 and FUS ALS models. Using a machine learning algorithm, healthy and aberrant neuromuscular synapses are identified with 95% accuracy, with 88% sensitivity and 97% specificity. Our results validate NMJ-Analyser as a robust platform for systematic and structural screening of NMJs, and pave the way for transferrable, and cross-comparison and high-throughput studies in neuromuscular diseases.

Terminal Schwann Cell Aging: Implications for Age-Associated Neuromuscular Dysfunction

Action potential is transmitted to muscle fibers through specialized synaptic interfaces called neuromuscular junctions (NMJs). These structures are capped by terminal Schwann cells (tSCs), which play essential roles during formation and maintenance of the NMJ. tSCs are implicated in the correct communication between nerves and muscles, and in reinnervation upon injury. During aging, loss of muscle mass and strength (sarcopenia and dynapenia) are due, at least in part, to the progressive loss of contacts between muscle fibers and nerves. Despite the important role of tSCs in NMJ function, very little is known on their implication in the NMJ-aging process and in age-associated denervation. This review summarizes the current knowledge about the implication of tSCs in the age-associated degeneration of NMJs. We also speculate on the possible mechanisms underlying the observed phenotypes.

Protein disulfide isomerase ERp57 protects early muscle denervation in experimental ALS

Amyotrophic lateral sclerosis (ALS) is a progressive fatal neurodegenerative disease that affects motoneurons. Mutations in superoxide dismutase 1 (SOD1) have been described as a causative genetic factor for ALS. Mice overexpressing ALS-linked mutant SOD1 develop ALS symptoms accompanied by histopathological alterations and protein aggregation. The protein disulfide isomerase family member ERp57 is one of the main up-regulated proteins in tissue of ALS patients and mutant SOD1 mice, whereas point mutations in ERp57 were described as possible risk factors to develop the disease. ERp57 catalyzes disulfide bond formation and isomerization in the endoplasmic reticulum (ER), constituting a central component of protein quality control mechanisms. However, the actual contribution of ERp57 to ALS pathogenesis remained to be defined. Here, we studied the consequences of overexpressing ERp57 in experimental ALS using mutant SOD1 mice. Double transgenic SOD1G93A/ERp57WT animals presented delayed deterioration of electrophysiological activity and maintained muscle innervation compared to single transgenic SOD1G93A littermates at early-symptomatic stage, along with improved motor performance without affecting survival. The overexpression of ERp57 reduced mutant SOD1 aggregation, but only at disease end-stage, dissociating its role as an anti-aggregation factor from the protection of neuromuscular junctions. Instead, proteomic analysis revealed that the neuroprotective effects of ERp57 overexpression correlated with increased levels of synaptic and actin cytoskeleton proteins in the spinal cord. Taken together, our results suggest that ERp57 operates as a disease modifier at early stages by maintaining motoneuron connectivity.

Whole-mount staining of neuromuscular junctions in adult mouse diaphragms with a sandwich-like apparatus

BACKGROUND

Investigation of neuromuscular junction (NMJ) morphology by immunochemistry can provide important insights into the physiological and pathological status of neuromuscular disorders. Sectioning and muscle fiber tearing are commonly required to prepare experimentally accessible samples, while muscles that are flat and thin can be investigated with whole-mount immunohistochemistry for a comprehensive overview of the entire innervation pattern. The diaphragm is important for respiratory function and one of the flat muscles frequently used for studying neuromuscular development as well as neuromuscular pathology. Nevertheless, techniques for reliable whole-mount immunolabeling of adult diaphragms are lacking, mainly due to the poor tissue permeability of labeling reagents. An effective approach for researchers to be able to comprehensively visualize and characterize NMJ defects of the adult diaphragm in mouse models is therefore of clear importance.

NEW METHOD

This protocol demonstrates that the diaphragm can be thinned and spread out under even pressure using two Perspex boards for better whole-mount immunostaining.

RESULTS

The expanded mouse diaphragm allows the comprehensive assessment of a number of NMJ phenotypes.

COMPARISON WITH EXISTING METHODS

Most peer-reviewed and online protocols can be applied to the embryonic diaphragm but fail to show the entire innervation pattern in the adult diaphragm. Our method provides a convenient approach and present a clear innervation pattern that increases the reliability of the assessment of NMJ phenotypes in the diaphragm.

CONCLUSIONS

This simple method for whole-mount immunostaining of the adult diaphragm will allow researchers to perform a detailed analysis of the neuromuscular system in mouse models.

Early motor deficits in the phalangeal fine movements induced by chronic AMPA infusion in the rat spinal cord assessed by a novel method: Phalangeal tension recording test

Motor behavior alterations are a shared hallmark of neurodegenerative diseases affecting motor circuits, such as amyotrophic lateral sclerosis (ALS), Parkinson's, and Huntington's diseases. In patients and transgenic animal models of amyotrophic lateral sclerosis fine movements controlled by distal muscles are the first to be affected, but its study and knowledge remain poorly understood, mainly because most of the tests used for describing the motor alterations are focused on the function of proximal muscles and gross movements. In this study we demonstrate that alterations of phalangeal fine movements can be quantitatively evaluated using a novel procedure designed by us, phalangeal tension recording test, which showed high sensitivity to detect such alterations. The evaluation was carried out during the motor neuron (MN) degenerative process induced by the acute and chronic overactivation of AMPA receptors in the lumbar rat spinal cord, using previously described models. The new method allowed the quantification of significant alterations of the fine movements of the hindpaws phalanges when AMPA was infused in the lumbar segment controlling the distal muscles, but not when a more rostral spinal segment was infused, and these alterations were not detected by the rotarod or the stride tests. These changes occurred before the paralysis of the hindlimbs. Studying the early distal motor alterations before the total paralysis at late stages is essential for understanding the initial consequences of MN degeneration and therefore for designing new strategies for the control, treatment and prevention of MN diseases.

Interaction of Axonal Chondrolectin with Collagen XIXa1 Is Necessary for Precise Neuromuscular Junction Formation

Chondrolectin (Chodl) is needed for motor axon extension in zebrafish and is dysregulated in mouse models of spinal muscular atrophy (SMA). However, the mechanistic basis of Chodl function is not known. Here, we use Chodl-deficient zebrafish and mouse mutants to show that the absence of Chodl leads to anatomical and functional defects of the neuromuscular synapse. In zebrafish, the growth of an identified motor axon beyond an "en passant" synapse and later axon branching from synaptic points are impaired, leading to functional deficits. Mechanistically, motor-neuron-autonomous Chodl function depends on its intracellular domain and on binding muscle-derived collagen XIXa1 by its extracellular C-type lectin domain. Our data support evolutionarily conserved roles of Chodl in synaptogenesis and provide evidence for a "synapse-first" scenario of motor axon growth in zebrafish.

Altered Sensory Neuron Development in CMT2D Mice Is Site-Specific and Linked to Increased GlyRS Levels

Dominant, missense mutations in the widely and constitutively expressed GARS1 gene cause peripheral neuropathy that usually begins in adolescence and principally impacts the upper limbs. Caused by a toxic gain-of-function in the encoded glycyl-tRNA synthetase (GlyRS) enzyme, the neuropathology appears to be independent of the canonical role of GlyRS in aminoacylation. Patients display progressive, life-long weakness and wasting of muscles in hands followed by feet, with frequently associated deficits in sensation. When dysfunction is observed in motor and sensory nerves, there is a diagnosis of Charcot-Marie-Tooth disease type 2D (CMT2D), or distal hereditary motor neuropathy type V if the symptoms are purely motor. The cause of this varied sensory involvement remains unresolved, as are the pathomechanisms underlying the selective neurodegeneration characteristic of the disease. We have previously identified in CMT2D mice that neuropathy-causing Gars mutations perturb sensory neuron fate and permit mutant GlyRS to aberrantly interact with neurotrophin receptors (Trks). Here, we extend this work by interrogating further the anatomy and function of the CMT2D sensory nervous system in mutant Gars mice, obtaining several key results: (1) sensory pathology is restricted to neurons innervating the hindlimbs; (2) perturbation of sensory development is not common to all mouse models of neuromuscular disease; (3) in vitro axonal transport of signaling endosomes is not impaired in afferent neurons of all CMT2D mouse models; and (4) Gars expression is selectively elevated in a subset of sensory neurons and linked to sensory developmental defects. These findings highlight the importance of comparative neurological assessment in mouse models of disease and shed light on key proposed neuropathogenic mechanisms in GARS1-linked neuropathy.

Developmental demands contribute to early neuromuscular degeneration in CMT2D mice

Dominantly inherited, missense mutations in the widely expressed housekeeping gene, GARS1, cause Charcot-Marie-Tooth type 2D (CMT2D), a peripheral neuropathy characterised by muscle weakness and wasting in limb extremities. Mice modelling CMT2D display early and selective neuromuscular junction (NMJ) pathology, epitomised by disturbed maturation and neurotransmission, leading to denervation. Indeed, the NMJ disruption has been reported in several different muscles; however, a systematic comparison of neuromuscular synapses from distinct body locations has yet to be performed. We therefore analysed NMJ development and degeneration across five different wholemount muscles to identify key synaptic features contributing to the distinct pattern of neurodegeneration in CMT2D mice. Denervation was found to occur along a distal-to-proximal gradient, providing a cellular explanation for the greater weakness observed in mutant Gars hindlimbs compared with forelimbs. Nonetheless, muscles from similar locations and innervated by axons of equivalent length showed significant differences in neuropathology, suggestive of additional factors impacting on site-specific neuromuscular degeneration. Defective NMJ development preceded and associated with degeneration, but was not linked to a delay of wild-type NMJ maturation processes. Correlation analyses indicate that muscle fibre type nor synaptic architecture explain the differential denervation of CMT2D NMJs, rather it is the extent of post-natal synaptic growth that predisposes to neurodegeneration. Together, this work improves our understanding of the mechanisms driving synaptic vulnerability in CMT2D and hints at pertinent pathogenic pathways.

TMEM16K is an interorganelle regulator of endosomal sorting

Communication between organelles is essential for their cellular homeostasis. Neurodegeneration reflects the declining ability of neurons to maintain cellular homeostasis over a lifetime, where the endolysosomal pathway plays a prominent role by regulating protein and lipid sorting and degradation. Here we report that TMEM16K, an endoplasmic reticulum lipid scramblase causative for spinocerebellar ataxia (SCAR10), is an interorganelle regulator of the endolysosomal pathway. We identify endosomal transport as a major functional cluster of TMEM16K in proximity biotinylation proteomics analyses. TMEM16K forms contact sites with endosomes, reconstituting split-GFP with the small GTPase RAB7. Our study further implicates TMEM16K lipid scrambling activity in endosomal sorting at these sites. Loss of TMEM16K function led to impaired endosomal retrograde transport and neuromuscular function, one of the symptoms of SCAR10. Thus, TMEM16K-containing ER-endosome contact sites represent clinically relevant platforms for regulating endosomal sorting.

A video protocol for rapid dissection of mouse dorsal root ganglia from defined spinal levels

OBJECTIVE

Dorsal root ganglia (DRG) are heterogeneous assemblies of assorted sensory neuron cell bodies found in bilateral pairs at every level of the spinal column. Pseudounipolar afferent neurons convert external stimuli from the environment into electrical signals that are retrogradely transmitted to the spinal cord dorsal horn. To do this, they extend single axons from their DRG-resident somas that then bifurcate and project both centrally and distally. DRG can be dissected from mice at embryonic stages and any age post-natally, and have been extensively used to study sensory neuron development and function, response to injury, and pathological processes in acquired and genetic diseases. We have previously published a step-by-step dissection method for the rapid isolation of post-natal mouse DRG. Here, the objective is to extend the protocol by providing training videos that showcase the dissection in fine detail and permit the extraction of ganglia from defined spinal levels.

RESULTS

By following this method, the reader will be able to swiftly and accurately isolate specific lumbar, thoracic, and cervical DRG from mice. Dissected ganglia can then be used for RNA/protein analyses, subjected to immunohistochemical examination, and cultured as explants or dissociated primary neurons, for in-depth investigations of sensory neuron biology.

Morphological variability is greater at developing than mature mouse neuromuscular junctions

The neuromuscular junction (NMJ) is the highly specialised peripheral synapse formed between lower motor neuron terminals and muscle fibres. Post‐synaptic acetylcholine receptors (AChRs), which are found in high density in the muscle membrane, bind to acetylcholine released into the synaptic cleft of the NMJ, thereby enabling the conversion of motor action potentials to muscle contractions. NMJs have been studied for many years as a general model for synapse formation, development and function, and are known to be early sites of pathological changes in many neuromuscular diseases. However, information is limited on the diversity of NMJs in different muscles, how synaptic morphology changes during development, and the relevance of these parameters to neuropathology. Here, this crucial gap was addressed using a robust and standardised semi‐automated workflow called NMJ‐morph to quantify features of pre‐ and post‐synaptic NMJ architecture in an unbiased manner. Five wholemount muscles from wild‐type mice were dissected and compared at immature (post‐natal day, P7) and early adult (P31−32) timepoints. The inter‐muscular variability was greater in mature post‐synaptic AChR morphology than that of the pre‐synaptic motor neuron terminal. Moreover, the developing NMJ showed greater differences across muscles than the mature synapse, perhaps due to the observed distinctions in synaptic growth between muscles. Nevertheless, the amount of nerve to muscle contact was consistent, suggesting that pathological denervation can be reliably compared across different muscles in mouse models of neurodegeneration. Additionally, mature post‐synaptic endplate diameters correlated with fibre type, independently of muscle fibre diameter. Altogether, this work provides detailed information on healthy pre‐ and post‐synaptic NMJ morphology from five anatomically and functionally distinct mouse muscles, delivering useful reference data for future comparison with neuromuscular disease models.

Neuronal over-expression of Oxr1 is protective against ALS-associated mutant TDP-43 mislocalisation in motor neurons and neuromuscular defects in vivo

A common pathological hallmark of amyotrophic lateral sclerosis (ALS) and the related neurodegenerative disorder frontotemporal dementia, is the cellular mislocalization of transactive response DNA-binding protein 43 kDa (TDP-43). Additionally, multiple mutations in the TARDBP gene (encoding TDP-43) are associated with familial forms of ALS. While the exact role for TDP-43 in the onset and progression of ALS remains unclear, the identification of factors that can prevent aberrant TDP-43 localization and function could be clinically beneficial. Previously, we discovered that the oxidation resistance 1 (Oxr1) protein could alleviate cellular mislocalization phenotypes associated with TDP-43 mutations, and that over-expression of Oxr1 was able to delay neuromuscular abnormalities in the hSOD1G93A ALS mouse model. Here, to determine whether Oxr1 can protect against TDP-43-associated phenotypes in vitro and in vivo, we used the same genetic approach in a newly described transgenic mouse expressing the human TDP-43 locus harbouring an ALS disease mutation (TDP-43M337V). We show in primary motor neurons from TDP-43M337V mice that genetically-driven Oxr1 over-expression significantly alleviates cytoplasmic mislocalization of mutant TDP-43. We also further quantified newly-identified, late-onset neuromuscular phenotypes of this mutant line, and demonstrate that neuronal Oxr1 over-expression causes a significant reduction in muscle denervation and neuromuscular junction degeneration in homozygous mutants in parallel with improved motor function and a reduction in neuroinflammation. Together these data support the application of Oxr1 as a viable and safe modifier of TDP-43-associated ALS phenotypes.

Synaptic withdrawal following nerve injury is influenced by postnatal maturity, muscle-specific properties, and the presence of underlying pathology in mice

Axonal and synaptic degeneration occur following nerve injury and during disease. Traumatic nerve injury results in rapid fragmentation of the distal axon and loss of synaptic terminals, in a process known as Wallerian degeneration (WD). Identifying and understanding factors that influence the rate of WD is of significant biological and clinical importance, as it will facilitate understanding of the mechanisms of neurodegeneration and identification of novel therapeutic targets. Here, we investigate levels of synaptic loss following nerve injury under a range of conditions, including during postnatal development, in a range of anatomically distinct muscles and in a mouse model of motor neuron disease. By utilising an ex vivo model of nerve injury, we show that synaptic withdrawal is slower during early postnatal development. Significantly more neuromuscular junctions remained fully innervated in the cranial nerve/muscle preparations analysed at P15 than at P25. Furthermore, we demonstrate variability in the level of synaptic withdrawal in response to injury in different muscles, with retraction being slower in abdominal preparations than in cranial muscles across all time points analysed. Importantly, differences between the cranial and thoracoabdominal musculature seen here are not consistent with differences in muscle vulnerability that have been previously reported in mouse models of the childhood motor neuron disease, spinal muscular atrophy (SMA), caused by depletion of survival motor neuron protein (Smn). To further investigate the relationship between synaptic degeneration in SMA and WD, we induced WD in preparations from the Smn^2B/− mouse model of SMA. In a disease‐resistant muscle (rostral band of levator auris longus), where there is minimal denervation, there was no change in the level of synaptic loss, which suggests that the process of synaptic withdrawal following injury is Smn‐independent. However, in a muscle with ongoing degeneration (transvs. abdominis), the level of synaptic loss significantly increased, with the percentage of denervated endplates increasing by 33% following injury, compared to disease alone. We therefore conclude that the presence of a die‐back can accelerate synaptic loss after injury in Smn^2B/− mice.

Loss of BICD2 in muscle drives motor neuron loss in a developmental form of spinal muscular atrophy

Autosomal dominant missense mutations in BICD2 cause Spinal Muscular Atrophy Lower Extremity Predominant 2 (SMALED2), a developmental disease of motor neurons. BICD2 is a key component of the cytoplasmic dynein/dynactin motor complex, which in axons drives the microtubule-dependent retrograde transport of intracellular cargo towards the cell soma. Patients with pathological mutations in BICD2 develop malformations of cortical and cerebellar development similar to Bicd2 knockout (-/-) mice. In this study we sought to re-examine the motor neuron phenotype of conditional Bicd2-/- mice. Bicd2-/- mice show a significant reduction in the number of large calibre motor neurons of the L4 ventral root compared to wild type mice. Muscle-specific knockout of Bicd2 results in a similar reduction in L4 ventral axons comparable to global Bicd2-/- mice. Rab6, a small GTPase required for the sorting of exocytic vesicles from the Trans Golgi Network to the plasma membrane is a major binding partner of BICD2. We therefore examined the secretory pathway in SMALED2 patient fibroblasts and demonstrated that BICD2 is required for physiological flow of constitutive secretory cargoes from the Trans Golgi Network to the plasma membrane using a VSV-G reporter assay. Together, these data indicate that BICD2 loss from muscles is a major driver of non-cell autonomous pathology in the motor nervous system, which has important implications for future therapeutic approaches in SMALED2.

Three-dimensional visualization of intramuscular innervation in intact adult skeletal muscle by a modified iDISCO method

Three-dimensional visualization of the innervation in skeletal muscles is helpful for understanding the morphological structure and function. iDISCO, a whole-mount immunolabeling and clearing technique, provides a valuable tool for volume imaging of intramuscular nerve fibers but suffers from the nonspecific staining caused by the anti-mouse secondary antibody when using the murine primary antibody. We developed a modified iDISCO method by introducing pretreatment of ScaleCUBIC-1 reagent, termed m-iDISCO. The m-iDISCO method could eliminate the nonspecific staining and achieve uniform and complete labeling of nerve fibers in various muscles with mouse anti-neurofilament primary antibody. Combining the m-iDISCO method with light-sheet microscopy enabled us to visualize the innervation of adult mouse tibialis anterior and trace the nerve fibers from extramuscular branches to intramuscular terminal branches. This method represents an effective alternative for studying the innervation of intact skeletal muscles in health and disease.

Organotypic Culture Assay for Neuromuscular Synaptic Degeneration and Function

We describe here an organotypic culture system we have used to investigate mechanisms that maintain structure and function of axon terminals at the neuromuscular junction (NMJ). We developed this by taking advantage of the slow Wallerian degeneration phenotype in mutant Wld^s mice, using these to compare preservation of NMJs with degeneration in nerve-muscle preparations from wild-type mice. We take hind limb tibial nerve/flexor digitorum brevis and lumbrical muscles and incubate them in mammalian physiological saline at 32 °C for 24-48 h. Integrity of NMJs can then be compared using a combination of electrophysiological and morphological techniques. We illustrate our method with data showing synaptic preservation ex vivo in nerve-muscle explants from Sarm-1 null-mutant mice. The ex vivo assays of NMJ integrity we describe here may therefore be useful for detailed investigation of synaptic maintenance and degeneration.

In Vivo Imaging of Anterograde and Retrograde Axonal Transport in Rodent Peripheral Nerves

Axonal transport, which is the process mediating the active shuttling of a variety cargoes from one end of an axon to the other, is essential for the development, function, and survival of neurons. Impairments in this dynamic process are linked to diverse nervous system diseases and advanced ageing. It is thus essential that we quantitatively study the kinetics of axonal transport to gain an improved understanding of neuropathology as well as the molecular and cellular mechanisms regulating cargo trafficking. One of the best ways to achieve this goal is by imaging individual, fluorescent cargoes in live systems and analyzing the kinetic properties of their progression along the axon. We have therefore developed an intravital technique to visualize different organelles, such as signaling endosomes and mitochondria, being actively transported in the axons of both motor and sensory neurons in live, anesthetized rodents. In this chapter, we provide step-by-step instructions on how to deliver specific organelle-targeting, fluorescent probes using several routes of administration to image individual cargoes being bidirectionally transported along axons within the exposed sciatic nerve. This method can provide detailed, physiologically relevant information on axonal transport, and is thus poised to elucidate mechanisms regulating this process in both health and disease.

Muscle Fiber Type Changes in Lumbrical Muscles at Early Stages of Chronic Nerve Compression

Chronic nerve compression (CNC) neuropathy is a common disease in the clinic and provokes paraesthesia, or numbness at early stage. The changes in muscle fiber composition and motor nerve terminal morphology in distal muscles were studied in this study. A well-established CNC model was used to assess the changes in the muscles. Behaviors were measured by von Frey filament test. The myosin heavy chain isoforms and neuromuscular junctions (NMJs) were stained by immunofluorescence to show the muscle fiber types composition and motor nerve terminals morphologic changes in the flexor digitorum longus (FDL) and lumbrical muscle. The fiber cross-sectional areas of different muscle fiber types were measured. The small-fiber degeneration of cutaneous nerve fibers was examined by detecting the protein gene product 9.5 (PGP9.5) with immunofluorescence. At 2nd month after compression, the proportion of type I and type II B fibers was markedly decreased, and that of type II A fibers was increased in the lumbrical muscle. There was no significant change in composition of muscle fiber types in FDL and NMJ morphology of FDL and lumbrical muscles. Intra-epidermal nerve fibre density (IENFD) declined at 2nd month after the compression. Our study reveals the morphological changes of the FDL and lumbrical muscle at an early stage of CNC. These findings may be helpful to understand muscle damage and pathophysiological development of the nerve compression, and provide new evidence for early treatment of CNC.

Inhibiting p38 MAPK alpha rescues axonal retrograde transport defects in a mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease caused by the degeneration of upper and lower motor neurons. Defects in axonal transport have been observed pre-symptomatically in the SOD1^G93A mouse model of ALS, and have been proposed to play a role in motor neuron degeneration as well as in other pathologies of the nervous system, such as Alzheimer's disease and hereditary neuropathies. In this study, we screen a library of small-molecule kinase inhibitors towards the identification of pharmacological enhancers of the axonal retrograde transport of signalling endosomes, which might be used to normalise the rate of this process in diseased neurons. Inhibitors of p38 mitogen-activated protein kinases (p38 MAPK) were identified in this screen and were found to correct deficits in axonal retrograde transport of signalling endosomes in cultured primary SOD1^G93A motor neurons. In vitro knockdown experiments revealed that the alpha isoform of p38 MAPK (p38 MAPKα) was the sole isoform responsible for SOD1^G93A-induced transport deficits. Furthermore, we found that acute treatment with p38 MAPKα inhibitors restored the physiological rate of axonal retrograde transport in vivo in early symptomatic SOD1^G93A mice. Our findings demonstrate the pathogenic effect of p38 MAPKα on axonal retrograde transport and identify a potential therapeutic strategy for ALS.

Developmental abnormality contributes to cortex-dependent motor impairments and higher intracortical current requirement in the reeler homozygous mutants

The motor deficit of the reeler mutants has largely been considered cerebellum related, and the developmental consequences of the cortex on reeler motor behavior have not been examined. We herein showed that there is a behavioral consequence to reeler mutation in models examined at cortex-dependent bimanual tasks that require forepaw dexterity. Using intracortical microstimulation, we found the forelimb representation in the motor cortex was significantly reduced in the reeler. The reeler cortex required a significantly higher current to evoke skeletal muscle movements, suggesting the cortical trans-synaptic propagation is disrupted. When the higher current was applied, the reeler motor representation was found preserved. To elucidate the influence of cerebellum atrophy and ataxia on the obtained results, the behavioral and neurophysiological findings in reeler mice were reproduced using the Disabled-1 (Dab1) cKO mice, in which the Reelin-Dab1 signal deficiency is confined to the cerebral cortex. The Dab1 cKO mice were further assessed at the single-pellet reach and retrieval task, displaying a lower number of successfully retrieved pellets. It suggests the abnormality confined to the cortex still reduced the dexterous motor performance. Although possible muscular dysfunction was reported in REELIN-deficient humans, the function of the reeler forelimb muscle examined by electromyography, morphology of neuromuscular junction and the expression level of choline acetyltransferase were normal. Our results suggest that the mammalian laminar structure is necessary for the forepaw skill performance and for trans-synaptic efficacy in the cortical output.

Imaging Analysis of the Neuromuscular Junction in Dystrophic Muscle

Duchenne muscular dystrophy (DMD), caused by the absence of the protein dystrophin, is characterized as a neuromuscular disease in which muscle weakness, increased susceptibility to muscle injury, and inadequate repair appear to underlie the pathology. Considerable attention has been dedicated to studying muscle fiber damage, but there is little information to determine if damage from contraction-induced injury also occurs at or near the nerve terminal axon. Interestingly, both human patients and the mouse model for DMD (the mdx mouse) present fragmented neuromuscular junction (NMJ) morphology. Studies of mdx mice have revealed presynaptic and postsynaptic abnormalities, nerve terminal discontinuity, as well as increased susceptibility of the NMJ to contraction-induced injury with corresponding functional changes in neuromuscular transmission and nerve-evoked electromyography. Focusing on the NMJ as a contributor to functional deficits in the muscle represents a paradigm shift from the more prevalent myocentric perspectives. Further studies are needed to determine the extent to which the nerve-muscle interaction is disrupted in DMD and the role of the NMJ in the dystrophic progression. This chapter lists the tools needed for nerve terminal and NMJ structural analysis using fluorescence imaging, and provides a step-by-step outline for how to stain, image, and analyze the NMJ in skeletal muscle, with specific attention to mdx muscle.

Developmental and adult-specific processes contribute to de novo neuromuscular regeneration in the lizard tail

Peripheral nerves exhibit robust regenerative capabilities in response to selective injury among amniotes, but the regeneration of entire muscle groups following volumetric muscle loss is limited in birds and mammals. In contrast, lizards possess the remarkable ability to regenerate extensive de novo muscle after tail loss. However, the mechanisms underlying reformation of the entire neuromuscular system in the regenerating lizard tail are not completely understood. We have tested whether the regeneration of the peripheral nerve and neuromuscular junctions (NMJs) recapitulate processes observed during normal neuromuscular development in the green anole, Anolis carolinensis. Our data confirm robust axonal outgrowth during early stages of tail regeneration and subsequent NMJ formation within weeks of autotomy. Interestingly, NMJs are overproduced as evidenced by a persistent increase in NMJ density 120 and 250 days post autotomy (DPA). Substantial Myelin Basic Protein (MBP) expression could also be detected along regenerating nerves indicating that the ability of Schwann cells to myelinate newly formed axons remained intact. Overall, our data suggest that the mechanism of de novo nerve and NMJ reformation parallel, in part, those observed during neuromuscular development. However, the prolonged increase in NMJ number and aberrant muscle differentiation hint at processes specific to the adult response. An examination of the coordinated exchange between peripheral nerves, Schwann cells, and newly synthesized muscle of the regenerating neuromuscular system may assist in the identification of candidate molecules that promote neuromuscular recovery in organisms incapable of a robust regenerative response.

Functional decline at the aging neuromuscular junction is associated with altered laminin-α4 expression

Laminin-α4 is involved in the alignment of active zones to postjunctional folds at the neuromuscular junction (NMJ). Prior study has implicated laminin-α4 in NMJ maintenance, with altered NMJ morphology observed in adult laminin-α4 deficient mice (lama4^−/−). The present study further investigated the role of laminin-α4 in NMJ maintenance by functional characterization of transmission properties, morphological investigation of synaptic proteins including synaptic laminin-α4, and neuromotor behavioral testing. Results showed maintained perturbed transmission properties at lama4^−/− NMJs from adult (3 months) through to aged (18-22 months). Hind-limb grip force demonstrated similar trends as transmission properties, with maintained weaker grip force across age groups in lama4^−/−. Interestingly, both transmission properties and hind-limb grip force in aged wild-types resembled those observed in adult lama4^−/−. Most significantly, altered expression of laminin-α4 was noted at the wild-type NMJs prior to the observed decline in transmission properties, suggesting that altered laminin-α4 expression precedes the decline of neurotransmission in aging wild-types. These findings significantly support the role of laminin-α4 in maintenance of the NMJ during aging.

Neuropilin 1 sequestration by neuropathogenic mutant glycyl-tRNA synthetase is permissive to vascular homeostasis

The mechanism by which dominantly inherited mutations in the housekeeping gene GARS, which encodes glycyl-tRNA synthetase (GlyRS), mediate selective peripheral nerve toxicity resulting in Charcot-Marie-Tooth disease type 2D (CMT2D) is still largely unresolved. The transmembrane receptor protein neuropilin 1 (Nrp1) was recently identified as an aberrant extracellular binding partner of mutant GlyRS. Formation of the Nrp1/mutant GlyRS complex antagonises Nrp1 interaction with one of its main natural ligands, vascular endothelial growth factor-A (VEGF-A), contributing to neurodegeneration. However, reduced extracellular binding of VEGF-A to Nrp1 is known to disrupt post-natal blood vessel development and growth. We therefore analysed the vascular system at early and late symptomatic time points in CMT2D mouse muscles, retina, and sciatic nerve, as well as in embryonic hindbrain. Mutant tissues show no difference in blood vessel diameter, density/growth, and branching from embryonic development to three months, spanning the duration over which numerous sensory and neuromuscular phenotypes manifest. Our findings indicate that mutant GlyRS-mediated disruption of Nrp1/VEGF-A signalling is permissive to maturation and maintenance of the vasculature in CMT2D mice.

NMJ-morph reveals principal components of synaptic morphology influencing structure-function relationships at the neuromuscular junction

The ability to form synapses is one of the fundamental properties required by the mammalian nervous system to generate network connectivity. Structural and functional diversity among synaptic populations is a key hallmark of network diversity, and yet we know comparatively little about the morphological principles that govern variability in the size, shape and strength of synapses. Using the mouse neuromuscular junction (NMJ) as an experimentally accessible model synapse, we report on the development of a robust, standardized methodology to facilitate comparative morphometric analysis of synapses ('NMJ-morph'). We used NMJ-morph to generate baseline morphological reference data for 21 separate pre- and post-synaptic variables from 2160 individual NMJs belonging to nine anatomically distinct populations of synapses, revealing systematic differences in NMJ morphology between defined synaptic populations. Principal components analysis revealed that overall NMJ size and the degree of synaptic fragmentation, alongside pre-synaptic axon diameter, were the most critical parameters in defining synaptic morphology. 'Average' synaptic morphology was remarkably conserved between comparable synapses from the left and right sides of the body. Systematic differences in synaptic morphology predicted corresponding differences in synaptic function that were supported by physiological recordings, confirming the robust relationship between synaptic size and strength.