The Role of Glial Cells in the Formation and Maintenance of the Neuromuscular Junction

Targeting neuromuscular junction to treat neuromuscular disorders

The integrity and preservation of the neuromuscular junction (NMJ), the interface between the motor neuron and skeletal muscle, is critical for maintaining a healthy skeletal muscle. The structural and/or functional defects in the three cellular components of NMJ, namely the pre-synaptic terminal, synaptic cleft, and post-synaptic region, negatively affect skeletal muscle mass and/or strength. Therefore, NMJ repair appears to be an appropriate therapy for muscle disorders. Mouse models provide a detailed molecular characterization of various cellular components of NMJ with relevance to human diseases. This review discusses different molecular targets on the three cellular components of NMJ for treating muscle diseases. The potential effects of these therapies on NMJ morphology and motor performance, their therapeutic efficacy, and clinical relevance are discussed. Collectively, the available data supports targeting NMJ alone or as an adjunct therapy in treating muscle disorders. However, the potential impact of such interventions on human patients with muscle disorders requires further investigation.

The MuSK-BMP pathway regulates synaptic Nav1.4 localization and muscle excitability

The neuromuscular junction (NMJ) is the linchpin of nerve-evoked muscle contraction. Broadly considered, the function of the NMJ is to transduce a nerve action potential into a muscle fiber action potential (MFAP). Efficient information transfer requires both cholinergic signaling, responsible for the generation of endplate potentials (EPPs), and excitation, the activation of postsynaptic voltage-gated sodium channels (Nav1.4) to trigger MFAPs. In contrast to the cholinergic apparatus, the signaling pathways that organize Nav1.4 and muscle fiber excitability are poorly characterized. Muscle-specific kinase (MuSK), in addition to its Ig1 domain-dependent role as an agrin-LRP4 receptor, is also a BMP co-receptor that binds BMPs via its Ig3 domain and shapes BMP-induced signaling and transcriptional output. Here we probed the function of the MuSK-BMP pathway at the NMJ using mice lacking the MuSK Ig3 domain ('ΔIg3-MuSK'). Synapses formed normally in ΔIg3-MuSK animals, but the postsynaptic apparatus was fragmented from the first weeks of life. Anatomical denervation was not observed at any age examined. Moreover, spontaneous and nerve-evoked acetylcholine release, AChR density, and endplate currents were comparable to WT. However, trains of nerve-evoked MFAPs in ΔIg3-MuSK muscle were abnormal as revealed by increased jitter and blocking in single fiber electromyography. Further, nerve-evoked compound muscle action potentials (CMAPs), as well as twitch and tetanic muscle torque force production, were also diminished. Finally, Nav1.4 levels were reduced at ΔIg3-MuSK synapses but not at the extrajunctional sarcolemma, indicating that the observed excitability defects are the result of impaired localization of this voltage-gated ion channel at the NMJ. We propose that MuSK plays two distinct roles at the NMJ: as an agrin-LRP4 receptor necessary for establishing and maintaining cholinergic signaling, and as a BMP co-receptor required for maintaining proper Nav1.4 density, nerve-evoked muscle excitability and force production. The MuSK-BMP pathway thus emerges as a target for modulating excitability and functional innervation, which are defective in conditions such as congenital myasthenic syndromes and aging.

TDP-43 dysregulation and neuromuscular junction disruption in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a disease characterized by upper and lower motor neuron (MN) loss with a signature feature of cytoplasmic aggregates containing TDP-43, which are detected in nearly all patients. Mutations in the gene that encodes TDP-43 (TARBDP) are known to result in both familial and sporadic ALS. In ALS, disruption of neuromuscular junctions (NMJs) constitutes a critical event in disease pathogenesis, leading to denervation atrophy, motor impairments and disability. Morphological defects and impaired synaptic transmission at NMJs have been reported in several TDP-43 animal models and in vitro, linking TDP-43 dysregulation to the loss of NMJ integrity in ALS. Through the lens of the dying-back and dying-forward hypotheses of ALS, this review discusses the roles of TDP-43 related to synaptic function, with a focus on the potential molecular mechanisms occurring within MNs, skeletal muscles and glial cells that may contribute to NMJ disruption in ALS.

Brainstem noradrenergic neurons: Identifying a hub at the intersection of cognition, motility, and skeletal muscle regulation

Brainstem noradrenergic neuron clusters form a node integrating efferents projecting to distinct areas such as those regulating cognition and skeletal muscle structure and function, and receive dissimilar afferents through established circuits to coordinate organismal responses to internal and environmental challenges. Genetic lineage tracing shows the remarkable heterogeneity of brainstem noradrenergic neurons, which may explain their varied functions. They project to the locus coeruleus, the primary source of noradrenaline in the brain, which supports learning and cognition. They also project to pre-ganglionic neurons, which lie within the spinal cord and form synapses onto post-ganglionic neurons. The synapse between descending brainstem noradrenergic neurons and pre-ganglionic spinal neurons, and these in turn with post-ganglionic noradrenergic neurons located at the paravertebral sympathetic ganglia, support an anatomical hierarchy that regulates skeletal muscle innervation, neuromuscular transmission, and muscle trophism. Whether any noradrenergic neuron subpopulation is more susceptible to damaged protein deposit and death with ageing and neurodegeneration is a relevant question that answer will help us to detect neurodegeneration at an early stage, establish prognosis, and anticipate disease progression. Loss of muscle mass and strength with ageing, termed sarcopenia, may predict impaired cognition with ageing and neurodegeneration and establish an early time to start interventions aimed at reducing central noradrenergic neurons hyperactivity. Complex multidisciplinary approaches, including genetic tracing, specific circuit labelling, optogenetics and chemogenetics, electrophysiology, and single-cell transcriptomics and proteomics, are required to test this hypothesis pre-clinical.

Structure and Function of the Mammalian Neuromuscular Junction

The mammalian neuromuscular junction (NMJ) comprises a presynaptic terminal, a postsynaptic receptor region on the muscle fiber (endplate), and the perisynaptic (terminal) Schwann cell. As with any synapse, the purpose of the NMJ is to transmit signals from the nervous system to muscle fibers. This neural control of muscle fibers is organized as motor units, which display distinct structural and functional phenotypes including differences in pre- and postsynaptic elements of NMJs. Motor units vary considerably in the frequency of their activation (both motor neuron discharge rate and duration/duty cycle), force generation, and susceptibility to fatigue. For earlier and more frequently recruited motor units, the structure and function of the activated NMJs must have high fidelity to ensure consistent activation and continued contractile response to sustain vital motor behaviors (e.g., breathing and postural balance). Similarly, for higher force less frequent behaviors (e.g., coughing and jumping), the structure and function of recruited NMJs must ensure short-term reliable activation but not activation sustained for a prolonged period in which fatigue may occur. The NMJ is highly plastic, changing structurally and functionally throughout the life span from embryonic development to old age. The NMJ also changes under pathological conditions including acute and chronic disease. Such neuroplasticity often varies across motor unit types. © 2022 American Physiological Society. Compr Physiol 12:1-36, 2022.

Heterogeneity and Potency of Peripheral Glial Cells in Embryonic Development and Adults

This review describes the heterogeneity of peripheral glial cell populations, from the emergence of Schwann cells (SCs) in early development, to their involvement, and that of their derivatives in adult glial populations. We focus on the origin of the first glial precursors from neural crest cells (NCCs), and their ability to differentiate into several cell types during development. We also discuss the heterogeneity of embryonic glia in light of the latest data from genetic tracing and transcriptome analysis. Special attention has been paid to the biology of glial populations in adult animals, by highlighting common features of different glial cell types and molecular differences that modulate their functions. Finally, we consider the communication of glial cells with axons of neurons in normal and pathological conditions. In conclusion, the present review details how information available on glial cell types and their functions in normal and pathological conditions may be utilized in the development of novel therapeutic strategies for the treatment of patients with neurodiseases.

Key concepts in muscle regeneration: muscle "cellular ecology" integrates a gestalt of cellular cross-talk, motility, and activity to remodel structure and restore function

This review identifies some key concepts of muscle regeneration, viewed from perspectives of classical and modern research. Early insights noted the pattern and sequence of regeneration across species was similar, regardless of the type of injury, and differed from epimorphic limb regeneration. While potential benefits of exercise for tissue repair was debated, regeneration was not presumed to deliver functional restoration, especially after ischemia-reperfusion injury; muscle could develop fibrosis and ectopic bone and fat. Standard protocols and tools were identified as necessary for tracking injury and outcomes. Current concepts vastly extend early insights. Myogenic regeneration occurs within the environment of muscle tissue. Intercellular cross-talk generates an interactive system of cellular networks that with the extracellular matrix and local, regional, and systemic influences, forms the larger gestalt of the satellite cell niche. Regenerative potential and adaptive plasticity are overlain by epigenetically regionalized responsiveness and contributions by myogenic, endothelial, and fibroadipogenic progenitors and inflammatory and metabolic processes. Muscle architecture is a living portrait of functional regulatory hierarchies, while cellular dynamics, physical activity, and muscle-tendon-bone biomechanics arbitrate regeneration. The scope of ongoing research-from molecules and exosomes to morphology and physiology-reveals compelling new concepts in muscle regeneration that will guide future discoveries for use in application to fitness, rehabilitation, and disease prevention and treatment.

Neuromuscular Junction Dysfunction in Amyotrophic Lateral Sclerosis

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

hiPSC-Derived Schwann Cells Influence Myogenic Differentiation in Neuromuscular Cocultures

Motoneurons, skeletal muscle fibers, and Schwann cells form synapses, termed neuromuscular junctions (NMJs). These control voluntary body movement and are affected in numerous neuromuscular diseases. Therefore, a variety of NMJ in vitro models have been explored to enable mechanistic and pharmacological studies. So far, selective integration of Schwann cells in these models has been hampered, due to technical limitations. Here we present robust protocols for derivation of Schwann cells from human induced pluripotent stem cells (hiPSC) and their coculture with hiPSC-derived motoneurons and C2C12 muscle cells. Upon differentiation with tuned BMP signaling, Schwann cells expressed marker proteins, S100b, Gap43, vimentin, and myelin protein zero. Furthermore, they displayed typical spindle-shaped morphologies with long processes, which often aligned with motoneuron axons. Inclusion of Schwann cells in coculture experiments with hiPSC-derived motoneurons and C2C12 myoblasts enhanced myotube growth and affected size and number of acetylcholine receptor plaques on myotubes. Altogether, these data argue for the availability of a consistent differentiation protocol for Schwann cells and their amenability for functional integration into neuromuscular in vitro models, fostering future studies of neuromuscular mechanisms and disease.

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.

T-box transcription factor 21 is expressed in terminal Schwann cells at the neuromuscular junction

INTRODUCTION/AIMS

Terminal Schwann cells (tSCs) are nonmyelinating Schwann cells present at the neuromuscular junction (NMJ) with multiple integral roles throughout their lifespan. There is no known gene differentiating tSCs from myelinating Schwann cells, making their isolation and investigation challenging. In this work we investigated genes expressed within tSCs.

METHODS

A novel dissection technique was utilized to isolate the tSC-containing NMJ band from the sternomastoid muscles of S100-GFP mice. RNA was isolated from samples containing: (a) NMJ bands (tSCs with nerve and muscle), (b) nerve, and (c) muscle, and microarray genetic expression analysis was conducted. Data were validated by quantitative real-time polymerase chain reaction (qRT-PCR) and immunofluorescent staining. To identify genes specific to tSCs compared with other NMJ components, analysis of variance and rank-order analysis were performed using the Partek Genomic Suite.

RESULTS

Microarray analysis of the tSC-enriched NMJ band revealed upregulation (by 4- to 12-fold) of several genes unique to the NMJ compared with muscle or nerve parts alone (P < .05). Among these genes, Tbx21 (or T-bet) was identified, which showed a 12-fold higher expression at the NMJ compared with sciatic nerve (P < .002). qRT-PCR analysis showed Tbx21 mRNA expression was over ninefold higher (P < .05) in the NMJ relative to muscle and nerve. Tbx21 protein colocalized with tSCs and was not noted in myelinating SCs from sciatic nerve.

DISCUSSION

We found TBX21 to be expressed in tSCs. Additional studies will be performed to determine the functional significance of TBX21 in tSCs. These studies may enhance the investigative tools available to modulate tSCs to improve motor recovery after nerve injury.

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.

Neuromuscular junction-on-a-chip: ALS disease modeling and read-out development in microfluidic devices

Amyotrophic lateral sclerosis (ALS) is a fatal and progressive neurodegenerative disease affecting upper and lower motor neurons with no cure available. Clinical and animal studies reveal that the neuromuscular junction (NMJ), a synaptic connection between motor neurons and skeletal muscle fibers, is highly vulnerable in ALS and suggest that NMJ defects may occur at the early stages of the disease. However, mechanistic insight into how NMJ dysfunction relates to the onset and progression of ALS is incomplete, which hampers therapy development. This is, in part, caused by a lack of robust in vitro models. The ability to combine microfluidic and induced pluripotent stem cell (iPSC) technologies has opened up new avenues for studying molecular and cellular ALS phenotypes in vitro. Microfluidic devices offer several advantages over traditional culture approaches when modeling the NMJ, such as the spatial separation of different cell types and increased control over the cellular microenvironment. Moreover, they are compatible with 3D cell culture, which enhances NMJ functionality and maturity. Here, we review how microfluidic technology is currently being employed to develop more reliable in vitro NMJ models. To validate and phenotype such models, various morphological and functional read-outs have been developed. We describe and discuss the relevance of these read-outs and specifically illustrate how these read-outs have enhanced our understanding of NMJ pathology in ALS. Finally, we share our view on potential future directions and challenges.

The Neuromuscular Junction in Health and Disease: Molecular Mechanisms Governing Synaptic Formation and Homeostasis

The neuromuscular junction (NMJ) is a highly specialized synapse between a motor neuron nerve terminal and its muscle fiber that are responsible for converting electrical impulses generated by the motor neuron into electrical activity in the muscle fibers. On arrival of the motor nerve action potential, calcium enters the presynaptic terminal, which leads to the release of the neurotransmitter acetylcholine (ACh). ACh crosses the synaptic gap and binds to ACh receptors (AChRs) tightly clustered on the surface of the muscle fiber; this leads to the endplate potential which initiates the muscle action potential that results in muscle contraction. This is a simplified version of the events in neuromuscular transmission that take place within milliseconds, and are dependent on a tiny but highly structured NMJ. Much of this review is devoted to describing in more detail the development, maturation, maintenance and regeneration of the NMJ, but first we describe briefly the most important molecules involved and the conditions that affect their numbers and function. Most important clinically worldwide, are myasthenia gravis (MG), the Lambert-Eaton myasthenic syndrome (LEMS) and congenital myasthenic syndromes (CMS), each of which causes specific molecular defects. In addition, we mention the neurotoxins from bacteria, snakes and many other species that interfere with neuromuscular transmission and cause potentially fatal diseases, but have also provided useful probes for investigating neuromuscular transmission. There are also changes in NMJ structure and function in motor neuron disease, spinal muscle atrophy and sarcopenia that are likely to be secondary but might provide treatment targets. The NMJ is one of the best studied and most disease-prone synapses in the nervous system and it is amenable to in vivo and ex vivo investigation and to systemic therapies that can help restore normal function.

Perisynaptic schwann cells - The multitasking cells at the developing neuromuscular junctions

Neuromuscular junctions (NMJs) are specialized synapses in the peripheral nervous system that allow the transmission of neuronal impulses to skeletal muscles for their contraction. Due to its size and accessibility, the NMJ is a commonly used model for studying basic principles of synapse organization and function. Similar to synapses in the central nervous system, NMJs are composed of presynaptic axonal terminals, the postsynaptic machinery formed at the membrane of the muscle fibers, and the synapse-associated glial cells. The special glial cells at the NMJs are called terminal Schwann cells or perisynaptic Schwann cells (PSCs). Decades of studies on the NMJ, as well as the most recent discoveries, have revealed multiple functions for PSCs at different stages of synaptic formation, maintenance, and disassembly. This review summarizes major observations in the field.

MMP-mediated modulation of ECM environment during axonal growth and NMJ development

Motor neurons, skeletal muscles, and perisynaptic Schwann cells interact with extracellular matrix (ECM) to form the tetrapartite synapse in the peripheral nervous system. Dynamic remodeling of ECM composition is essential to diversify its functions for distinct cellular processes during neuromuscular junction (NMJ) development. In this review, we give an overview of the proteolytic regulation of ECM proteins, particularly by secreted and membrane-type matrix metalloproteinases (MMPs), in axonal growth and NMJ development. It is suggested that MMP-2, MMP-9, and membrane type 1-MMP (MT1-MMP) promote axonal outgrowth and regeneration upon injury by clearing the glial scars at the lesion site. In addition, these MMPs also play roles in neuromuscular synaptogenesis, ranging from spontaneous formation of synaptic specializations to activity-dependent synaptic elimination, via proteolytic cleavage or degradation of growth factors, neurotrophic factors, and ECM molecules. For instance, secreted MMP-3 has been known to cleave agrin, the main postsynaptic differentiation inducer, further highlighting the importance of MMPs in NMJ formation and maintenance. Furthermore, the increased level of several MMPs in myasthenia gravis (MG) patient suggest the pathophysiological mechanisms of MMP-mediated proteolytic degradation in MG pathogenesis. Finally, we speculate on potential major future directions for studying the regulatory functions of MMP-mediated ECM remodeling in axonal growth and NMJ development.

Neuromuscular Junction as an Entity of Nerve-Muscle Communication

One of the crucial systems severely affected in several neuromuscular diseases is the loss of effective connection between muscle and nerve, leading to a pathological non-communication between the two tissues. The neuromuscular junction (NMJ) represents the critical region at the level of which muscle and nerve communicate. Defects in signal transmission between terminal nerve endings and muscle membrane is a common feature of several physio-pathologic conditions including aging and Amyotrophic Lateral Sclerosis (ALS). Nevertheless, controversy exists on whether pathological events beginning at the NMJ precede or follow loss of motor units. In this review, the role of NMJ in the physio-pathologic interplay between muscle and nerve is discussed.

The sympathetic nervous system regulates skeletal muscle motor innervation and acetylcholine receptor stability

AIM

Symptoms of autonomic failure are frequently the presentation of advanced age and neurodegenerative diseases that impair adaptation to common physiologic stressors. The aim of this work was to examine the interaction between the sympathetic and motor nervous system, the involvement of the sympathetic nervous system (SNS) in neuromuscular junction (NMJ) presynaptic motor function, the stability of postsynaptic molecular organization, and the skeletal muscle composition and function.

METHODS

Since muscle weakness is a symptom of diseases characterized by autonomic dysfunction, we studied the impact of regional sympathetic ablation on muscle motor innervation by using transcriptome analysis, retrograde tracing of the sympathetic outflow to the skeletal muscle, confocal and electron microscopy, NMJ transmission by electrophysiological methods, protein analysis, and state of the art microsurgical techniques, in C57BL6, MuRF1KO and Thy-1 mice.

RESULTS

We found that the SNS regulates motor nerve synaptic vesicle release, skeletal muscle transcriptome, muscle force generated by motor nerve activity, axonal neurofilament phosphorylation, myelin thickness, and myofibre subtype composition and CSA. The SNS also modulates the levels of postsynaptic membrane acetylcholine receptor by regulating the Gαi2 -Hdac4-Myogenin-MuRF1pathway, which is prevented by the overexpression of the guanine nucleotide-binding protein Gαi2 (Q205L), a constitutively active mutant G protein subunit.

CONCLUSION

The SNS regulates NMJ transmission, maintains optimal Gαi2 expression, and prevents any increase in Hdac4, myogenin, MuRF1, and miR-206. SNS ablation leads to upregulation of MuRF1, muscle atrophy, and downregulation of postsynaptic AChR. Our findings are relevant to clinical conditions characterized by progressive decline of sympathetic innervation, such as neurodegenerative diseases and aging.

Direct conversion of adult human skin fibroblasts into functional Schwann cells that achieve robust recovery of the severed peripheral nerve in rats

Direct conversion is considered a promising approach to obtain tissue-specific cells for cell therapies; however, this strategy depends on exogenous gene expression that may cause undesired adverse effects such as tumorigenesis. By optimizing the Schwann cell induction system, which was originally developed for trans-differentiation of bone marrow mesenchymal stem cells into Schwann cells, we established a system to directly convert adult human skin fibroblasts into cells comparable to authentic human Schwann cells without gene introduction. Serial treatments with beta-mercaptoethanol, retinoic acid, and finally a cocktail of basic fibroblast growth factor, forskolin, platelet-derived growth factor-AA, and heregulin-β1 (EGF domain) converted fibroblasts into cells expressing authentic Schwann cell markers at an efficiency of approximately 75%. Genome-wide gene expression analysis suggested the conversion of fibroblasts into the Schwann cell-lineage. Transplantation of induced Schwann cells into severed peripheral nerve of rats facilitated axonal regeneration and robust functional recovery in sciatic function index comparable to those of authentic human Schwann cells. The contributions of induced Schwann cells to myelination of regenerated axons and re-formation of neuromuscular junctions were also demonstrated. Our data clearly demonstrated that cells comparable to functional Schwann cells feasible for the treatment of neural disease can be induced from adult human skin fibroblasts without gene introduction. This direct conversion system will be beneficial for clinical applications to peripheral and central nervous system injuries and demyelinating diseases.

Clinical relevance of terminal Schwann cells: An overlooked component of the neuromuscular junction

The terminal Schwann cell (tSC), a type of nonmyelinating Schwann cell, is a significant yet relatively understudied component of the neuromuscular junction. In addition to reviewing the role tSCs play on formation, maintenance, and remodeling of the synapse, we review studies that implicate tSCs in neuromuscular diseases including spinal muscular atrophy, Miller-Fisher syndrome, and amyotrophic lateral sclerosis, among others. We also discuss the importance of these cells on degeneration and regeneration after nerve injury. Knowledge of tSC biology may improve our understanding of disease pathogenesis and help us identify new and innovative therapeutic strategies for the many patients who suffer from neuromuscular disorders and nerve injuries.

Amyotrophic Lateral Sclerosis (ALS) and Adenosine Receptors

In the present review we discuss the potential involvement of adenosinergic signaling, in particular the role of adenosine receptors, in amyotrophic lateral sclerosis (ALS). Though the literature on this topic is not abundant, the information so far available on adenosine receptors in animal models of ALS highlights the interest to continue to explore the role of these receptors in this neurodegenerative disease. Indeed, all motor neurons affected in ALS are responsive to adenosine receptor ligands but interestingly, there are alterations in pre-symptomatic or early symptomatic stages that mirror those in advanced disease stages. Information starts to emerge pointing toward a beneficial role of A_2A receptors (A_2AR), most probably at early disease states, and a detrimental role of caffeine, in clear contrast with what occurs in other neurodegenerative diseases. However, some evidence also exists on a beneficial action of A_2AR antagonists. It may happen that there are time windows where A_2AR prove beneficial and others where their blockade is required. Furthermore, the same changes may not occur simultaneously at the different synapses. In line with this, it is not fully understood if ALS is a dying back disease or if it propagates in a centrifugal way. It thus seems crucial to understand how motor neuron dysfunction occurs, how adenosine receptors are involved in those dysfunctions and whether the early changes in purinergic signaling are compensatory or triggers for the disease. Getting this information is crucial before starting the design of purinergic based strategies to halt or delay disease progression.

Choline transporter mutations in severe congenital myasthenic syndrome disrupt transporter localization

The presynaptic, high-affinity choline transporter is a critical determinant of signalling by the neurotransmitter acetylcholine at both central and peripheral cholinergic synapses, including the neuromuscular junction. Here we describe an autosomal recessive presynaptic congenital myasthenic syndrome presenting with a broad clinical phenotype due to homozygous choline transporter missense mutations. The clinical phenotype ranges from the classical presentation of a congenital myasthenic syndrome in one patient (p.Pro210Leu), to severe neurodevelopmental delay with brain atrophy (p.Ser94Arg) and extend the clinical outcomes to a more severe spectrum with infantile lethality (p.Val112Glu). Cells transfected with mutant transporter construct revealed a virtually complete loss of transport activity that was paralleled by a reduction in transporter cell surface expression. Consistent with these findings, studies to determine the impact of gene mutations on the trafficking of the Caenorhabditis elegans choline transporter orthologue revealed deficits in transporter export to axons and nerve terminals. These findings contrast with our previous findings in autosomal dominant distal hereditary motor neuropathy of a dominant-negative frameshift mutation at the C-terminus of choline transporter that was associated with significantly reduced, but not completely abrogated choline transporter function. Together our findings define divergent neuropathological outcomes arising from different classes of choline transporter mutation with distinct disease processes and modes of inheritance. These findings underscore the essential role played by the choline transporter in sustaining acetylcholine neurotransmission at both central and neuromuscular synapses, with important implications for treatment and drug selection.

Schwann Cells in Neuromuscular Junction Formation and Maintenance

The neuromuscular junction (NMJ) is a tripartite synapse that is formed by motor nerve terminals, postjunctional muscle membranes, and terminal Schwann cells (TSCs) that cover the nerve-muscle contact. NMJ formation requires intimate communications among the three different components. Unlike nerve-muscle interaction, which has been well characterized, less is known about the role of SCs in NMJ formation and maintenance. We show that SCs in mice lead nerve terminals to prepatterned AChRs. Ablating SCs at E8.5 (i.e., prior nerve arrival at the clusters) had little effect on aneural AChR clusters at E13.5, suggesting that SCs may not be necessary for aneural clusters. SC ablation at E12.5, a time when phrenic nerves approach muscle fibers, resulted in smaller and fewer nerve-induced AChR clusters; however, SC ablation at E15.5 reduced AChR cluster size but had no effect on cluster density, suggesting that SCs are involved in AChR cluster maturation. Miniature endplate potential amplitude, but not frequency, was reduced when SCs were ablated at E15.5, suggesting that postsynaptic alterations may occur ahead of presynaptic deficits. Finally, ablation of SCs at P30, after NMJ maturation, led to NMJ fragmentation and neuromuscular transmission deficits. Miniature endplate potential amplitude was reduced 3 d after SC ablation, but both amplitude and frequency were reduced 6 d after. Together, these results indicate that SCs are not only required for NMJ formation, but also necessary for its maintenance; and postsynaptic function and structure appeared to be more sensitive to SC ablation.

SIGNIFICANCE STATEMENT

Neuromuscular junctions (NMJs) are critical for survival and daily functioning. Defects in NMJ formation during development or maintenance in adulthood result in debilitating neuromuscular disorders. The role of Schwann cells (SCs) in NMJ formation and maintenance was not well understood. We genetically ablated SCs during development and after NMJ formation to investigate the consequences of the ablation. This study reveals a critical role of SCs in NMJ formation as well as maintenance.

A Novel Egr-1-Agrin Pathway and Potential Implications for Regulation of Synaptic Physiology and Homeostasis at the Neuromuscular Junction

Synaptic transmission requires intricate coordination of the components involved in processing of incoming signals, formation and stabilization of synaptic machinery, neurotransmission and in all related signaling pathways. Changes to any of these components cause synaptic imbalance and disruption of neuronal circuitry. Extensive studies at the neuromuscular junction (NMJ) have greatly aided in the current understanding of synapses and served to elucidate the underlying physiology as well as associated adaptive and homeostatic processes. The heparan sulfate proteoglycan agrin is a vital component of the NMJ, mediating synaptic formation and maintenance in both brain and muscle, but very little is known about direct control of its expression. Here, we investigated the relationship between agrin and transcription factor early growth response-1 (Egr-1), as Egr-1 regulates the expression of many genes involved in synaptic homeostasis and plasticity. Using chromatin immunoprecipitation (ChIP), cell culture with cell lines derived from brain and muscle, and animal models, we show that Egr-1 binds to the AGRN gene locus and suppresses its expression. When compared with wild type (WT), mice deficient in Egr-1 (Egr-1−/−) display a marked increase in AGRN mRNA and agrin full-length and cleavage fragment protein levels, including the 22 kDa, C-terminal fragment in brain and muscle tissue homogenate. Because agrin is a crucial component of the NMJ, we explored possible physiological implications of the Egr-1-agrin relationship. In the diaphragm, Egr-1−/− mice display increased NMJ motor endplate density, individual area and area of innervation. In addition to increased density, soleus NMJs also display an increase in fragmented and faint endplates in Egr-1−/− vs. WT mice. Moreover, the soleus NMJ electrophysiology of Egr-1−/− mice revealed increased quantal content and motor testing showed decreased movement and limb muscle strength compared with WT. This study provides evidence for the potential involvement of a novel Egr-1-agrin pathway in synaptic homeostatic and compensatory mechanisms at the NMJ. Synaptic homeostasis is greatly affected by the process of aging. These and other data suggest that changes in Egr-1 expression may directly or indirectly promote age-related pathologies.

Macrophage-mediated inflammation and glial response in the skeletal muscle of a rat model of familial amyotrophic lateral sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by progressive motor dysfunction and loss of large motor neurons in the spinal cord and brain stem. While much research has focused on mechanisms of motor neuron cell death in the spinal cord, degenerative processes in skeletal muscle and neuromuscular junctions (NMJs) are also observed early in disease development. Although recent studies support the potential therapeutic benefits of targeting the skeletal muscle in ALS, relatively little is known about inflammation and glial responses in skeletal muscle and near NMJs, or how these responses contribute to motor neuron survival, neuromuscular innervation, or motor dysfunction in ALS. We recently showed that human mesenchymal stem cells modified to release glial cell line-derived neurotrophic factor (hMSC-GDNF) extend survival and protect NMJs and motor neurons in SOD1(G93A) rats when delivered to limb muscles. In this study, we evaluate inflammatory and glial responses near NMJs in the limb muscle collected from a rat model of familial ALS (SOD1(G93A) transgenic rats) during disease progression and following hMSC-GDNF transplantation. Muscle samples were collected from pre-symptomatic, symptomatic, and end-stage animals. A significant increase in the expression of microglial inflammatory markers (CD11b and CD68) occurred in the skeletal muscle of symptomatic and end-stage SOD1(G93A) rats. Inflammation was confirmed by ELISA for inflammatory cytokines interleukin-1 β (IL-1β) and tumor necrosis factor-α (TNF-α) in muscle homogenates of SOD1(G93A) rats. Next, we observed active glial responses in the muscle of SOD1(G93A) rats, specifically near intramuscular axons and NMJs. Interestingly, strong expression of activated glial markers, glial fibrillary acidic protein (GFAP) and nestin, was observed in the areas adjacent to NMJs. Finally, we determined whether ex vivo trophic factor delivery influences inflammation and terminal Schwann cell (TSC) response during ALS. We found that intramuscular transplantation of hMSC-GDNF tended to exhibit less inflammation and significantly maintained TSC association with NMJs. Understanding cellular responses near NMJs is important to identify suitable cellular and molecular targets for novel treatment of ALS and other neuromuscular diseases.

Congenital Myasthenic Syndrome Type 19 Is Caused by Mutations in COL13A1, Encoding the Atypical Non-fibrillar Collagen Type XIII α1 Chain

The neuromuscular junction (NMJ) consists of a tripartite synapse with a presynaptic nerve terminal, Schwann cells that ensheathe the terminal bouton, and a highly specialized postsynaptic membrane. Synaptic structural integrity is crucial for efficient signal transmission. Congenital myasthenic syndromes (CMSs) are a heterogeneous group of inherited disorders that result from impaired neuromuscular transmission, caused by mutations in genes encoding proteins that are involved in synaptic transmission and in forming and maintaining the structural integrity of NMJs. To identify further causes of CMSs, we performed whole-exome sequencing (WES) in families without an identified mutation in known CMS-associated genes. In two families affected by a previously undefined CMS, we identified homozygous loss-of-function mutations in COL13A1, which encodes the alpha chain of an atypical non-fibrillar collagen with a single transmembrane domain. COL13A1 localized to the human muscle motor endplate. Using CRISPR-Cas9 genome editing, modeling of the COL13A1 c.1171delG (p.Leu392Sfs^∗71) frameshift mutation in the C2C12 cell line reduced acetylcholine receptor (AChR) clustering during myotube differentiation. This highlights the crucial role of collagen XIII in the formation and maintenance of the NMJ. Our results therefore delineate a myasthenic disorder that is caused by loss-of-function mutations in COL13A1, encoding a protein involved in organization of the NMJ, and emphasize the importance of appropriate symptomatic treatment for these individuals.

Terminal Schwann Cells Lead Synapse Remodelling following Injury(1,2)

This commentary article describes the importance and significance of the article recently published by the Kang and colleagues in The Journal of Neuroscience in 2014. Kang and colleagues provided new features of injury induced synapse remodelling.

Mechanisms Regulating Neuromuscular Junction Development and Function and Causes of Muscle Wasting

The neuromuscular junction is the chemical synapse between motor neurons and skeletal muscle fibers. It is designed to reliably convert the action potential from the presynaptic motor neuron into the contraction of the postsynaptic muscle fiber. Diseases that affect the neuromuscular junction may cause failure of this conversion and result in loss of ambulation and respiration. The loss of motor input also causes muscle wasting as muscle mass is constantly adapted to contractile needs by the balancing of protein synthesis and protein degradation. Finally, neuromuscular activity and muscle mass have a major impact on metabolic properties of the organisms. This review discusses the mechanisms involved in the development and maintenance of the neuromuscular junction, the consequences of and the mechanisms involved in its dysfunction, and its role in maintaining muscle mass during aging. As life expectancy is increasing, loss of muscle mass during aging, called sarcopenia, has emerged as a field of high medical need. Interestingly, aging is also accompanied by structural changes at the neuromuscular junction, suggesting that the mechanisms involved in neuromuscular junction maintenance might be disturbed during aging. In addition, there is now evidence that behavioral paradigms and signaling pathways that are involved in longevity also affect neuromuscular junction stability and sarcopenia.

Early and persistent abnormal decoding by glial cells at the neuromuscular junction in an ALS model

Amyotrophic lateral sclerosis (ALS) is a late-onset neuromuscular disease characterized by progressive loss of motor neurons (MNs) preceded by neuromuscular junction (NMJ) denervation. Despite the importance of NMJ denervation in ALS, the mechanisms involved remain unexplored and ill defined. The contribution of glial cells in the disease has been highlighted, including axonal Schwann cell activation that precedes the decline of motor function and the onset of hindlimb paralysis. Because NMJ denervation occurs early in the process and that perisynaptic Schwann cells (PSCs), glial cells at the NMJ, regulate morphological stability, integrity, and repair of the NMJ, one could predict that PSC functions would be altered even before denervation, contributing to NMJ malfunctions. We tested this possibility using a slowly progressive model of ALS (SOD1(G37R) mice). We observed a normal NMJ organization at a presymptomatic stage of ALS (120 d), but PSC detection of endogenous synaptic activity revealed by intracellular Ca(2+) changes was enhanced compared with their wild-type littermates. This inappropriate PSC decoding ability was associated with an increased level of neurotransmitter release and dependent on intrinsic glial properties related to enhanced muscarinic receptor activation. The alteration of PSC muscarinic receptor functions also persists during the preonset stage of the disease and became dependent on MN vulnerability with age. Together, these results suggest that PSC properties are altered in the disease process in a manner that would be detrimental for NMJ repair. The impairments of PSC functions may contribute to NMJ dysfunction and ALS pathogenesis.

LRP4 is critical for neuromuscular junction maintenance

The neuromuscular junction (NMJ) is a synapse between motor neurons and skeletal muscle fibers, and is critical for control of muscle contraction. Its formation requires neuronal agrin that acts by binding to LRP4 to stimulate MuSK. Mutations have been identified in agrin, MuSK, and LRP4 in patients with congenital myasthenic syndrome, and patients with myasthenia gravis develop antibodies against agrin, LRP4, and MuSK. However, it remains unclear whether the agrin signaling pathway is critical for NMJ maintenance because null mutation of any of the three genes is perinatal lethal. In this study, we generated imKO mice, a mutant strain whose LRP4 gene can be deleted in muscles by doxycycline (Dox) treatment. Ablation of the LRP4 gene in adult muscle enabled studies of its role in NMJ maintenance. We demonstrate that Dox treatment of P30 mice reduced muscle strength and compound muscle action potentials. AChR clusters became fragmented with diminished junctional folds and synaptic vesicles. The amplitude and frequency of miniature endplate potentials were reduced, indicating impaired neuromuscular transmission and providing cellular mechanisms of adult LRP4 deficiency. We showed that LRP4 ablation led to the loss of synaptic agrin and the 90 kDa fragments, which occurred ahead of other prejunctional and postjunctional components, suggesting that LRP4 may regulate the stability of synaptic agrin. These observations demonstrate that LRP4 is essential for maintaining the structural and functional integrity of the NMJ and that loss of muscle LRP4 in adulthood alone is sufficient to cause myasthenic symptoms.

The craniosacral progression of muscle development influences the emergence of neuromuscular junction alterations in a severe murine model for spinal muscular atrophy

AIMS

As 4-day-old mice of the severe spinal muscular atrophy (SMA) model (dying at 5-8 days) display pronounced neuromuscular changes in the diaphragm but not the soleus muscle, we wanted to gain more insight into the relationship between muscle development and the emergence of pathological changes and additionally to analyse intercostal muscles which are affected in human SMA.

METHODS

Structures of muscle fibres and neuromuscular junctions (NMJs) of the diaphragm, intercostal and calf muscles of prenatal (E21) and postnatal (P0 and P4) healthy and SMA mice were analysed by light and transmission electron microscopy. NMJ innervation was studied by whole mount immunofluorescence in diaphragms of P4 mice.

RESULTS

During this period, the investigated muscles still show a significant neck-to-tail developmental gradient. The diaphragm and calf muscles are most and least advanced, respectively, with respect to muscle fibre fusion and differentiation. The number and depth of subsynaptic folds increases, and perisynaptic Schwann cells (PSCs) acquire a basal lamina on their outer surface. Subsynaptic folds are connected to an extensive network of tubules and beaded caveolae, reminiscent of the T system in adult muscle. Interestingly, intercostal muscles from P4 SMA mice show weaker pathological involvement (that is, vacuolization of PSCs and perineurial cells) than those previously described by us for the diaphragm, whereas calf muscles show no pathological changes.

CONCLUSION

SMA-related alterations appear to occur only when the muscles have reached a certain developmental maturity. Moreover, glial cells, in particular PSCs, play an important role in SMA pathogenesis.

Mitochondrial alarmins released by degenerating motor axon terminals activate perisynaptic Schwann cells

An acute and highly reproducible motor axon terminal degeneration followed by complete regeneration is induced by some animal presynaptic neurotoxins, representing an appropriate and controlled system to dissect the molecular mechanisms underlying degeneration and regeneration of peripheral nerve terminals. We have previously shown that nerve terminals exposed to spider or snake presynaptic neurotoxins degenerate as a result of calcium overload and mitochondrial failure. Here we show that toxin-treated primary neurons release signaling molecules derived from mitochondria: hydrogen peroxide, mitochondrial DNA, and cytochrome c. These molecules activate isolated primary Schwann cells, Schwann cells cocultured with neurons and at neuromuscular junction in vivo through the MAPK pathway. We propose that this inter- and intracellular signaling is involved in triggering the regeneration of peripheral nerve terminals affected by other forms of neurodegenerative diseases.

Clinical implications of Schwann cell biology

The neuroglia of the peripheral nervous system (PNS) are derived from the neural crest and are a diverse family of cells. They consist of myelinating Schwann cells, non-myelinating Schwann cells, satellite cells, and perisynaptic Schwann cells. Due to their prominent role in the formation of myelin, myelinating Schwann cells are the best recognised of these cells. However, Schwann cells and the other neuroglia of the PNS have many functions that are independent of myelination and contribute significantly to the functioning of the peripheral nerve in both health and disease. Here we discuss the contribution of PNS neuroglial cells to clinical deficit in neurodegenerative disease, peripheral neuropathy, and pain.

Axonal regeneration after sciatic nerve lesion is delayed but complete in GFAP- and vimentin-deficient mice

Peripheral axotomy of motoneurons triggers Wallerian degeneration of injured axons distal to the lesion, followed by axon regeneration. Centrally, axotomy induces loss of synapses (synaptic stripping) from the surface of lesioned motoneurons in the spinal cord. At the lesion site, reactive Schwann cells provide trophic support and guidance for outgrowing axons. The mechanisms of synaptic stripping remain elusive, but reactive astrocytes and microglia appear to be important in this process. We studied axonal regeneration and synaptic stripping of motoneurons after a sciatic nerve lesion in mice lacking the intermediate filament (nanofilament) proteins glial fibrillary acidic protein (GFAP) and vimentin, which are upregulated in reactive astrocytes and Schwann cells. Seven days after sciatic nerve transection, ultrastructural analysis of synaptic density on the somata of injured motoneurons revealed more remaining boutons covering injured somata in GFAP(-/-)Vim(-/-) mice. After sciatic nerve crush in GFAP(-/-)Vim(-/-) mice, the fraction of reinnervated motor endplates on muscle fibers of the gastrocnemius muscle was reduced 13 days after the injury, and axonal regeneration and functional recovery were delayed but complete. Thus, the absence of GFAP and vimentin in glial cells does not seem to affect the outcome after peripheral motoneuron injury but may have an important effect on the response dynamics.

Morphological characteristics of motor neurons do not determine their relative susceptibility to degeneration in a mouse model of severe spinal muscular atrophy

Spinal muscular atrophy (SMA) is a leading genetic cause of infant mortality, resulting primarily from the degeneration and loss of lower motor neurons. Studies using mouse models of SMA have revealed widespread heterogeneity in the susceptibility of individual motor neurons to neurodegeneration, but the underlying reasons remain unclear. Data from related motor neuron diseases, such as amyotrophic lateral sclerosis (ALS), suggest that morphological properties of motor neurons may regulate susceptibility: in ALS larger motor units innervating fast-twitch muscles degenerate first. We therefore set out to determine whether intrinsic morphological characteristics of motor neurons influenced their relative vulnerability to SMA. Motor neuron vulnerability was mapped across 10 muscle groups in SMA mice. Neither the position of the muscle in the body, nor the fibre type of the muscle innervated, influenced susceptibility. Morphological properties of vulnerable and disease-resistant motor neurons were then determined from single motor units reconstructed in Thy.1-YFP-H mice. None of the parameters we investigated in healthy young adult mice - including motor unit size, motor unit arbor length, branching patterns, motor endplate size, developmental pruning and numbers of terminal Schwann cells at neuromuscular junctions - correlated with vulnerability. We conclude that morphological characteristics of motor neurons are not a major determinant of disease-susceptibility in SMA, in stark contrast to related forms of motor neuron disease such as ALS. This suggests that subtle molecular differences between motor neurons, or extrinsic factors arising from other cell types, are more likely to determine relative susceptibility in SMA.

The Onecut transcription factor HNF-6 regulates in motor neurons the formation of the neuromuscular junctions

The neuromuscular junctions are the specialized synapses whereby spinal motor neurons control the contraction of skeletal muscles. The formation of the neuromuscular junctions is controlled by a complex interplay of multiple mechanisms coordinately activated in motor nerve terminals and in their target myotubes. However, the transcriptional regulators that control in motor neurons the genetic programs involved in neuromuscular junction development remain unknown. Here, we provide evidence that the Onecut transcription factor HNF-6 regulates in motor neurons the formation of the neuromuscular junctions. Indeed, adult Hnf6 mutant mice exhibit hindlimb muscle weakness and abnormal locomotion. This results from defects of hindlimb neuromuscular junctions characterized by an abnormal morphology and defective localization of the synaptic vesicle protein synaptophysin at the motor nerve terminals. These defects are consequences of altered and delayed formation of the neuromuscular junctions in newborn mutant animals. Furthermore, we show that the expression level of numerous regulators of neuromuscular junction formation, namely agrin, neuregulin-2 and TGF-ß receptor II, is downregulated in the spinal motor neurons of Hnf6 mutant newborn animals. Finally, altered formation of neuromuscular junction-like structures in a co-culture model of wildtype myotubes with mutant embryonic spinal cord slices is rescued by recombinant agrin and neuregulin, indicating that depletion in these factors contributes to defective neuromuscular junction development in the absence of HNF-6. Thus, HNF-6 controls in spinal motor neurons a genetic program that coordinates the formation of hindlimb neuromuscular junctions.

Neuromuscular junction protection for the potential treatment of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neuromuscular disease characterized by the progressive degeneration of upper and lower motor neurons (MNs), leading to muscular atrophy and eventual respiratory failure. ALS research has primarily focused on mechanisms regarding MN cell death; however, degenerative processes in the skeletal muscle, particularly involving neuromuscular junctions (NMJs), are observed in the early stages of and throughout disease progression. According to the "dying-back" hypothesis, NMJ degeneration may not only precede, but actively cause upper and lower MN loss. The importance of NMJ pathology has relatively received little attention in ALS, possibly because compensatory mechanisms mask NMJ loss for prolonged periods. Many mechanisms explaining NMJ degeneration have been proposed such as the disruption of anterograde/retrograde axonal transport, irregular cellular metabolism, and changes in muscle gene and protein expression. Neurotrophic factors, which are known to have neuroprotective and regenerative properties, have been intensely investigated for their therapeutic potential in both the preclinical and clinical setting. Additional research should focus on the potential of preserving NMJs in order to delay or prevent disease progression.

Wnt signaling in neuromuscular junction development

Wnt proteins are best known for their profound roles in cell patterning, because they are required for the embryonic development of all animal species studied to date. Besides regulating cell fate, Wnt proteins are gaining increasing recognition for their roles in nervous system development and function. New studies indicate that multiple positive and negative Wnt signaling pathways take place simultaneously during the formation of vertebrate and invertebrate neuromuscular junctions. Although some Wnts are essential for the formation of NMJs, others appear to play a more modulatory role as part of multiple signaling pathways. Here we review the most recent findings regarding the function of Wnts at the NMJ from both vertebrate and invertebrate model systems.

Molecular mechanisms underlying maturation and maintenance of the vertebrate neuromuscular junction

The vertebrate neuromuscular junction (NMJ), a peripheral synapse formed between motoneuron and skeletal muscle, is characterized by a protracted postnatal period of maturation and life-long maintenance. In neuromuscular disorders such as congenital myasthenic syndromes (CMSs), disruptions of NMJ maturation and/or maintenance are frequently observed. In particular, defective neuromuscular transmission associated with structural and molecular abnormalities at the pre- and postsynaptic membranes, as well as at the synaptic cleft, has been reported in these patients. Here, we review recent advances in the understanding of molecular and cellular events that mediate NMJ maturation and maintenance. The underlying regulatory mechanisms, including key molecular regulators at the presynaptic nerve terminal, synaptic cleft, and postsynaptic muscle membrane, are discussed.

Neuron-glia interactions: the roles of Schwann cells in neuromuscular synapse formation and function

The NMJ (neuromuscular junction) serves as the ultimate output of the motor neurons. The NMJ is composed of a presynaptic nerve terminal, a postsynaptic muscle and perisynaptic glial cells. Emerging evidence has also demonstrated an existence of perisynaptic fibroblast-like cells at the NMJ. In this review, we discuss the importance of Schwann cells, the glial component of the NMJ, in the formation and function of the NMJ. During development, Schwann cells are closely associated with presynaptic nerve terminals and are required for the maintenance of the developing NMJ. After the establishment of the NMJ, Schwann cells actively modulate synaptic activity. Schwann cells also play critical roles in regeneration of the NMJ after nerve injury. Thus, Schwann cells are indispensable for formation and function of the NMJ. Further examination of the interplay among Schwann cells, the nerve and the muscle will provide insights into a better understanding of mechanisms underlying neuromuscular synapse formation and function.

Wnt4 participates in the formation of vertebrate neuromuscular junction

Neuromuscular junction (NMJ) formation requires the highly coordinated communication of several reciprocal signaling processes between motoneurons and their muscle targets. Identification of the early, spatially restricted cues in target recognition at the NMJ is still poorly documented, especially in mammals. Wnt signaling is one of the key pathways regulating synaptic connectivity. Here, we report that Wnt4 contributes to the formation of vertebrate NMJ in vivo. Results from a microarray screen and quantitative RT-PCR demonstrate that Wnt4 expression is regulated during muscle cell differentiation in vitro and muscle development in vivo, being highly expressed when the first synaptic contacts are formed and subsequently downregulated. Analysis of the mouse Wnt4−/− NMJ phenotype reveals profound innervation defects including motor axons overgrowing and bypassing AChR aggregates with 30% of AChR clusters being unapposed by nerve terminals. In addition, loss of Wnt4 function results in a 35% decrease of the number of prepatterned AChR clusters while Wnt4 overexpression in cultured myotubes increases the number of AChR clusters demonstrating that Wnt4 directly affects postsynaptic differentiation. In contrast, muscle structure and the localization of several synaptic proteins including acetylcholinesterase, MuSK and rapsyn are not perturbed in the Wnt4 mutant. Finally, we identify MuSK as a Wnt4 receptor. Wnt4 not only interacts with MuSK ectodomain but also mediates MuSK activation. Taken together our data reveal a new role for Wnt4 in mammalian NMJ formation that could be mediated by MuSK, a key receptor in synaptogenesis.

Growth factor delivery systems and repair strategies for damaged peripheral nerves

Artificial nerve conduits (NCs) are, in certain cases, instrumental for repairing damaged peripheral nerves, although therapeutic efficacy remains often suboptimal. Considerable efforts have been made to improve the therapeutic performance of NCs. This article reviews recent developments in NC-technology for peripheral nerve regeneration with a main focus on growth factors delivery systems and repair strategies. Commonly used materials for NC fabrication include collagen, silk fibroin, and biodegradable aliphatic polyesters. The basic NC structure, i.e., a hollow tube, can be manufactured by diverse methods: spinning mandrel technology, sheet rolling, injection-molding, freeze-drying, and electro-spinning. Polymeric and cellular delivery systems for growth factors can be integrated in the NC wall or within luminal structures such as gels, fibers, or biological materials providing binding affinity for the bioactive compounds. NCs with sustained growth factor delivery generally improve significantly the axonal outgrowth in nerve defect models, although restoration of sensory and motor functions remains inferior to that achieved with autologous nerve grafts. To improve therapeutic outcomes, further biofunctionalization of NCs will be needed, i.e., adjusting degradation kinetics of NC scaffolding to be compatible with axonal regeneration; delivering multiple growth factors at individually optimized kinetics; incorporating modality specific glial cells and furnishing NCs with guiding nanostructures.

Collagen XXVIII is a distinctive component of the peripheral nervous system nodes of ranvier and surrounds nonmyelinating glial cells

Growing evidence indicates that collagens perform crucial functions during the development and organization of the nervous system. Collagen XXVIII is a recently discovered collagen almost exclusively expressed in the peripheral nervous system (PNS). In this study, we show that this collagen is associated with nonmyelinated regions of the PNS. With the notable exception of type II terminal Schwann cell in the hairy skin, collagen XXVIII surrounds all nonmyelinating glial cells studied. This includes satellite glial cells of the dorsal root ganglia, terminal Schwann cells type I around mechanoceptors in the skin, terminal Schwann cells around proprioceptors in the muscle spindle or at the neuromuscular junction and olfactory ensheathing cells. Collagen XXVIII is also detected at nodes of Ranvier where the myelin sheath of myelinated fibers is interrupted and is thus a distinctive component of the PNS nodal gap. The correlation between the absence of myelin and the presence of collagen XXVIII is confirmed in a mouse model of Charcot-Marie-Tooth characterized by dysmyelinated nerve fibers, in which enhancement of collagen XXVIII labeling is observed.

Age affects reciprocal cellular interactions in neuromuscular synapses following peripheral nerve injury

Studies of the influence of age on regeneration and reinnervation in the peripheral nervous system (PNS) and neuromuscular junction (NMJ) are reviewed, with a particular focus on aged and denervated skeletal muscles. The morphological and functional features of incomplete regeneration and reinnervation are compared between adult and aged animals. In addition, some possible mechanisms of the age-related defects will be discussed. Increased fragmentation or damage in individual components of the NMJ (terminal Schwann cells (TSCs), axon terminals and acetylcholine receptor sites occurs during muscle reinnervation following PNS injury in the aged animals. The capacity to produce ultraterminal sprouting or multiple innervation secondary to PNS injury is maintained, but not the capacity to eliminate such anomalous axonal profiles. The frequency and accuracy of reoccupation of the synaptic sites by TSCs and axon terminals are impaired. Thus, despite the capability of extending neural processes, the rate at which regenerating nerve fibers grow, mature and precisely appose the postsynaptic muscle fiber is impaired, resulting in the failure of re-establishment of the normal single motor innervation in the NMJ. A complex set of cellular interactions in the NMJ are known to participate in the neurotrophism and neurotrophism to support growth of the regenerating and sprouting axons and their pathfinding to direct the target muscle fiber. Besides the capability of α-motoneurons, signaling originating from the TSCs and muscle may be impaired during aging.

Synapse-glia interactions are governed by synaptic and intrinsic glial properties

It is believed that glial cell activation and their interactions with synapses are predominantly dependent upon the characteristics of synaptic activity and the level of transmitter release. Because synaptic properties vary from one type of synapse to another, synapse-glia interactions should differ accordingly. The goal of this work was to examine how glial cell activation is dependent upon the properties of their respective synapses as well as the level of synaptic activity. We contrasted Ca(2+) responses of perisynaptic Schwann cells (PSCs) at neuromuscular junctions (NMJs) with different synaptic properties; the slow-twitch soleus (SOL) and the fast-twitch levator auris longus (LAL) muscles. Amplitude of PSC Ca(2+) responses elicited by repeated motor nerve stimulation at 40, 50 and 100 Hz were larger and their kinetics faster at LAL NMJs and this, at all frequencies examined. In addition, a greater number of PSCs per NMJ was activated by sustained synaptic transmission at NMJs of LAL in comparison to SOL. Differences in PSC activation could not be explained solely by differences in levels of transmitter release but also by intrinsic PSC properties since increasing transmitter release with tetraethylammonium chloride (TEA) did not increase their responsiveness. As a whole, these results indicate that PSC responsiveness at NMJs of slow- and fast-twitch muscles differ not only according to the level of activity of their synaptic partner but also in accordance with inherent glial properties.

Contributions of astrocytes to synapse formation and maturation - Potential functions of the perisynaptic extracellular matrix

The concept of the tripartite synapse proposes that in addition to the presynapse and the postsynaptic membrane closely apposed processes of astrocytes constitute an integral part of the synapse. Accordingly, astrocytes may influence synaptic activity by various ways. Thus glia- and neuron-derived neurotrophins, cytokines and metabolites influence neuronal survival, synaptic activity and plasticity. Beyond these facts, the past years have shown that astrocytes are required for synaptogenesis, the structural maintenance and proper functioning of synapses. In particular, astrocytes seem to play a key role in the organization of the brain's extracellular matrix (ECM) - most prominently the so-called perineuronal nets (PNNs), complex macromolecular assemblies of ECM components. Due to progress in cellular and molecular neurosciences, it has been possible to decipher the composition of ECM structures and to obtain insight into their function(s) and underlying mechanisms. It appears that PNN-related structures are involved in regulating the sprouting and pruning of synapses, which represents an important morphological correlate of synaptic plasticity in the adult nervous system. Perturbation assays and gene elimination by recombinant techniques have provided clear indications that astrocyte-derived ECM components, e.g. the tenascins and chondroitinsulfate proteoglycans (CSPGs) of the lectican family participate in these biological functions. The present review will discuss the glia-derived glycoproteins and CSPGs of the perisynaptic ECM, their neuronal and glial receptors, and in vitro assays to test their physiological functions in the framework of the synapse, the pivotal element of communication in the central nervous system.

Role of glial cells in the formation and maintenance of synapses

Synaptogenesis is a decisive process for the development of the brain, its plasticity during adulthood and its regeneration after injury and disease. Despite tremendous progress during the last decades, it remains unclear, whether neurons can form synapses autonomously. In this review, I will summarize recent evidence that this is probably not the case and that distinct phases of synapse development depend on help from glial cells. The results supporting this view come from studies on the central and peripheral nervous system and on different experimental models including cultured cells as well as living flies, worms and mice. Our understanding of synapse-glia interactions in the developing, adult and diseased brain is likely to advance more rapidly as new experimental approaches to identify, visualize and manipulate glial cells in vivo become available.