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Rescue of neuromuscular synapses by unplugged/MuSK. (A) Rescue of neural synapses at different induction times by Tg(hsp70l:unplugged FL-myc). Embryos were heat-shocked starting at the indicated time points, fixed at 27hpf, and stained for motor axons (znp-1, green) and AChR clusters (a-BTX, red). Individual hemisegments were scored as 'good rescue' (when AChR clusters colocalized with presynaptic nerve ending on most muscle fibers, including along ectopic nerve ending), 'mild rescue' (when synapses formed on ,50% of all muscles), or 'weak rescue' (when synapses formed only on non-migratory adaxial cells). 20 hemisegments were scored in each embryo. Results were summarized from two independent experiments (n = 1602380, average = 280, hemisegments per time point). (B) Tg(hsp70l:unplugged FL-myc) rescues neuromuscular synapses at 28 hpf and 48 hpf. Embryos were given a 40-min HS at 28hpf or 48hpf, and examined 30 minutes after the HS treatment. (B1-B6) In transgenic embryos, neuromuscular synapses formed on most muscle fibers and aligned with nerve ending. Arrows point to unplugged-characteristic pathfinding errors, indicating that the HS was too late to rescue axon pathfinding. (B7-B12) The single HS treatment at 48 hpf induced synapses in transgenic embryos (arrowheads in B11 and B12), albeit synapses appear smaller than those in wildtype. Inset in each panel is the enlarged image of the neuromuscular synapse pointed by the arrow. Scale bars: 20 mM. doi:10.1371/journal.pone.0008843.g004
Source publication
One of the earliest events in neuromuscular junction (NMJ) development is the accumulation of acetylcholine receptor (AChR) at the center of muscle cells. The unplugged/MuSK (muscle specific tyrosine kinase) gene is essential to initiate AChR clustering but also to restrict approaching growth cones to the muscle center, thereby coordinating pre- an...
Contexts in source publication
Context 1
... this set of experiments, embryos from Tg(hsp70l:unplugged FL-myc); unplugged br307/br307 animals crossed to unplugged br307/br307 animals were HS treated starting at different times points, and the presence of neuromuscular synapses was scored at 27 hpf (see Materials and Methods for HS treatments). At this time point, AChR clusters are detectable in the center of each muscle fiber where they form focal synapses, as well as aneural AChR at somite boundaries, where once the nerve has extended, myoseptal synapses will form [5,19]. Individual somitic hemisegments were scored as good, mild or weak rescue depending on the number and size of AChRs clusters that were colocalized with nerve terminals ( Figure 4A, see Figure 4 legends for each category of rescue; aneural AChR clusters at the somite boundaries were not included in this analysis). Despite a slight decrease over time, the rescue efficacy of Tg(hsp70l:unplugged FL- myc) was similar between the 5-somite stage and 24 hpf. ...
Context 2
... this set of experiments, embryos from Tg(hsp70l:unplugged FL-myc); unplugged br307/br307 animals crossed to unplugged br307/br307 animals were HS treated starting at different times points, and the presence of neuromuscular synapses was scored at 27 hpf (see Materials and Methods for HS treatments). At this time point, AChR clusters are detectable in the center of each muscle fiber where they form focal synapses, as well as aneural AChR at somite boundaries, where once the nerve has extended, myoseptal synapses will form [5,19]. Individual somitic hemisegments were scored as good, mild or weak rescue depending on the number and size of AChRs clusters that were colocalized with nerve terminals ( Figure 4A, see Figure 4 legends for each category of rescue; aneural AChR clusters at the somite boundaries were not included in this analysis). Despite a slight decrease over time, the rescue efficacy of Tg(hsp70l:unplugged FL- myc) was similar between the 5-somite stage and 24 hpf. ...
Context 3
... a slight decrease over time, the rescue efficacy of Tg(hsp70l:unplugged FL- myc) was similar between the 5-somite stage and 24 hpf. To determine how late during development unplugged/MuSK signaling is competent to induce neuromuscular synapses, embryos were HS treated once at 28 hpf or at 48 hpf, and analyzed 30 minutes after HS treatment ( Figure 4B). In transgenic embryos which were HS treated at 28 hpf, most if not all neuromuscular synapses were restored, with the majority of postsynaptic AChR clusters correctly aligning with the presynaptic nerve terminals (Figure 4B5-B6). ...
Context 4
... determine how late during development unplugged/MuSK signaling is competent to induce neuromuscular synapses, embryos were HS treated once at 28 hpf or at 48 hpf, and analyzed 30 minutes after HS treatment ( Figure 4B). In transgenic embryos which were HS treated at 28 hpf, most if not all neuromuscular synapses were restored, with the majority of postsynaptic AChR clusters correctly aligning with the presynaptic nerve terminals (Figure 4B5-B6). Surprisingly, in transgenic embryos HS treated at 48 hpf, many AChR clusters were present, and properly aligned along most, if not all, nerve endings (arrowheads in Figure 4B11-B12). ...
Similar publications
The neuromuscular junction (NMJ) is a cholinergic synapse that rapidly conveys signals from motoneurons to muscle cells and exhibits a high degree of subcellular specialization characteristic of chemical synapses. NMJ formation requires agrin and its coreceptors LRP4 and MuSK. Increasing evidence indicates that Wnt signaling regulates NMJ formation...
The accumulation of acetylcholine receptors (AChRs) at nerve terminals is critical for signal transmission at the neuromuscular junction, and rapsyn is essential for this process. Previous studies suggest that AChRs might direct rapsyn self-clusters to the synapse. In vivo experiments with fluorescently tagged AChR or rapsyn in zebrafish larvae rev...
The formation and function of synapses are tightly orchestrated by the precise timing of expression of specific molecules during development. In this study, we determined how manipulating the timing of expression of postsynaptic acetylcholine receptors (AChRs) impacts presynaptic release by establishing a genetically engineered zebrafish line in wh...
Skeletal muscle fibers are generally classified into two groups: slow (type I) and fast (type II). Fibers in each group are uniquely designed for specific locomotory needs based on their intrinsic cellular properties and the types of motor neurons that innervate them. In this review, we will focus on the current concept of slow muscle fibers which,...
This review provides an overview of zebrafish mutants with dysfunctional acetylcholine receptors or related proteins at the neuromuscular junction (NMJ). The NMJ, which has served as the classical model of the chemical synapse, uses acetylcholine as the neurotransmitter, and mutations of proteins involved in the signaling cascade lead to a variety...
Citations
... The ability to perform high-resolution imaging in a living organism has revolutionized the field of biological research, and the zebrafish is playing a critical role in advancing our understanding of the underlying mechanisms of development and disease. Several studies have capitalized on the advantages of the zebrafish and provided important insights in motor axonal growth (Myers et al. 1986, Chen et al. 2012, Sainath and Granato 2013, neuromuscular junction (NMJ) development (Jing et al. 2010, Bailey et al. 2019, synaptogenesis (Jontes et al. 2000, Hutson and Chien 2002, Panzer et al. 2005, Son et al. 2020, muscle development (Skobo et al. 2014, Dubi nska-Magiera 2016, Chen et al. 2017, Yin et al. 2022, and in NMJ deficits underlying neuromuscular pathologies (Patten et al. 2017, Butti et al. 2021, Lescouzères et al. 2023. In the last few decades, the zebrafish has particularly emerged as a valuable tool in studying the pathogenesis and development of therapeutic interventions for various neuromuscular disorders (Singh and Patten 2022). ...
Motivation
Neuromuscular junction structural integrity is crucial for transducing motoneuron signals that initiate skeletal muscle contraction. Zebrafish has emerged as a simple and efficient model to study neuromuscular junction structural morphology and function in the context of developmental neurobiology and neuromuscular diseases (NMDs). However, methods to quantify NMJ morphology from voluminous data of NMJ confocal images accurately, rapidly and reproducibly are lacking.
Results
We developed an ImageJ macro called “NMJ Analyser” to automatically and unbiasedly analyse NMJ morphology in zebrafish. From the Z-stack of a zebrafish hemi-somite, both presynaptic and postsynaptic fluorescently labelled termini at NMJs are extracted from background signal, with larger clusters of termini being segmented into individual termini using an unbiased algorithm. The program then determines whether each presynaptic terminus is colocalized with a postsynaptic terminus and vice versa, or whether it is orphaned, and tabulates the number of orphan and colocalized pre- and postsynaptic termini. The usefulness of this ImageJ macro plugin will be helpful to quantify NMJ parameters in zebrafish, particularly during development and in disease models of NMDs. It can enable high-throughput NMJ phenotypic screens in the drug discovery process for NMDs. It could also be further applied to the investigation of NMJ of other developmental systems.
Availability
NMJ Analyser is available for download at https://github.com/PattenLab/NMJ-Analyser.git
Supplementary information
Supplementary data are available at Bioinformatics online.
... The crucial role of Agrin-Lrp4-MuSK signaling in NMJ development has been studied mainly in a small range of muscle types such as axial muscle in zebrafish or murine diaphragm muscle, which differ in fiber type composition [6,74,[76][77][78][79]. Recently, Walker et al. analyzed the larval pectoral fin of zebrafish and revealed a new aspect of Agrin-Lrp4 signaling involved in MuSK-dependent synapse formation. ...
Muscle-specific kinase (MuSK) is the key regulator of neuromuscular junction development. MuSK acts via several distinct pathways and is responsible for pre- and postsynaptic differentiation. MuSK is unique among receptor tyrosine kinases as activation and signaling are particularly tightly regulated. Initiation of kinase activity requires Agrin, a heparan sulphate proteoglycan derived from motor neurons, the low-density lipoprotein receptor-related protein-4 (Lrp4) and the intracellular adaptor protein Dok-7. There is a great knowledge gap between MuSK activation and downstream signaling. Recent studies using omics techniques have addressed this knowledge gap, thereby greatly contributing to a better understanding of MuSK signaling. Impaired MuSK signaling causes severe muscle weakness as described in congenital myasthenic syndromes or myasthenia gravis but the underlying pathophysiology is often unclear. This review focuses on recent advances in deciphering MuSK activation and downstream signaling. We further highlight latest break-throughs in understanding and treatment of MuSK-related disorders and discuss the role of MuSK in non-muscle tissue.
... The communication between the cranial motor neurons and the muscle fibers are mediated through specialized synaptic association called neuromuscular junctions (NMJs). NMJs comprise of the 1) presynaptic nerve terminals formed by the axonal end, 2) the synaptic cleft where the neurotransmitter acetylcholine (ACh) is released and 3) the postsynaptic muscle endplate with aggregated acetylcholine receptors (AChRs) (Jing et al., 2010). To test if ethanol disrupts NMJs, we investigated the distribution of synaptic junctions in the branchial musculature of zebrafish exposed to 1% ethanol from 6hpf to 4dpf. ...
... A series of reciprocal interactions between the innervating axonal fiber and the muscle endplate is necessary to induce NMJs in their precise locations. The Agrin-muscle-specific kinase (MUSK)-lowdensity lipoprotein receptor-related protein 4 (Lrp4) signaling pathway plays a key role in establishing the NMJ, particularly clustering AChRs in the postsynaptic termini (Jing et al., 2010). Ethanol has been shown to reduce agrin-induced acetylcholine receptor clustering in C2C12 cells (Owen et al., 2010). ...
Forming a vertebrate head involves the meticulous integration of multiple tissue types during development. Prenatal alcohol exposure is known to cause a variety of birth defects, especially to tissues in the vertebrate head. However, a systematic analysis of coordinated defects across tissues in the head is lacking. Here, we delineate the effects of ethanol on individual tissue types and their integration during craniofacial development. We found that exposure to 1% ethanol induced ectopic cranial muscle and nerve defects with only slight effects on skeletal pattern. Ectopic muscles were, however, unaccompanied by ectopic tendons and could be partially rescued by anesthetizing the larvae before muscle fibers appeared. This finding suggests that the ectopic muscles result from fiber detachment and are not due to an underlying muscle patterning defect. Interestingly, immobilization did not rescue the nerve defects, thus ethanol has an independent effect on each tissue even though they are linked in developmental time and space. Time-course experiments demonstrated an increase in nerve defects with ethanol exposure between 48hpf-4dpf. Time-lapse imaging confirmed the absence of nerve pathfinding or misrouting defects until 48hpf. These results indicate that ethanol-induced nerve defects occur at the time of muscle innervation and after musculoskeletal patterning. Further, we investigated the effect of ethanol on the neuromuscular junctions of the craniofacial muscles and found a reduced number of postsynaptic receptors with no significant effect on the presynaptic terminals. Our study shows that craniofacial soft tissues are particularly susceptible to ethanol-induced damage and that these defects appear independent from one another. Thus, the effects of ethanol on the vertebrate head appear highly pleiotropic.
... The locomotion-sustaining human neuronal circuitry, which is highly conserved in zebrafish, was characterized, thanks to this species (Kimmel et al., 1995;Beattie, 2000). In particular, studies using this model contributed to the identification of molecular signals required for the establishment of motor networks (Berg et al., 2018), neuromuscular junction (NMJ) development, NMJ maintenance, synaptogenesis (Jing et al., 2009(Jing et al., , 2010Banerjee et al., 2011), and identification of muscle precursor types (Devoto et al., 1996). ...
... Many molecular, histological, and ultrastructural features of NMJ development and integrity are well conserved between mammals and zebrafish (Swenarchuk, 2019). Among other examples, the key role of AChR pre-patterning in guiding MN terminals to the muscle fibers was discovered in the zebrafish model (Panzer et al., 2005), as well as the role of the Wnt pathway in this process (Jing et al., 2010). ...
The zebrafish (Danio rerio) is a vertebrate species offering multitude of advantages for the study of conserved biological systems in human and has considerably enriched our knowledge in developmental biology and physiology. Being equally important in medical research, the zebrafish has become a critical tool in the fields of diagnosis, gene discovery, disease modeling, and pharmacology-based therapy. Studies on the zebrafish neuromuscular system allowed for deciphering key molecular pathways in this tissue, and established it as a model of choice to study numerous motor neurons, neuromuscular junctions, and muscle diseases. Starting with the similarities of the zebrafish neuromuscular system with the human system, we review disease models associated with the neuromuscular system to focus on current methodologies employed to study them and outline their caveats. In particular, we put in perspective the necessity to develop standardized and high-resolution methodologies that are necessary to deepen our understanding of not only fundamental signaling pathways in a healthy tissue but also the changes leading to disease phenotype outbreaks, and offer templates for high-content screening strategies. While the development of high-throughput methodologies is underway for motility assays, there is no automated approach to quantify the key molecular cues of the neuromuscular junction. Here, we provide a novel high-throughput imaging methodology in the zebrafish that is standardized, highly resolutive, quantitative, and fit for drug screening. By providing a proof of concept for its robustness in identifying novel molecular players and therapeutic drugs in giant axonal neuropathy (GAN) disease, we foresee that this new tool could be useful for both fundamental and biomedical research.
... erc1b À/À mutants are small and diffusely distributed in muscle tissue NMJ formation and clustering is well established, especially for spinal motor neurons in zebrafish 1,11,42,43 ; however, innervation of the lower jaw muscles by cranial nerves has received considerably less attention. We evaluated cranial nerve innervation of jaw muscles by wholemount IF staining of NMJs with an antibody against SV2, which labels presynaptic terminals in the cranial nerves, and a fluorescently conjugated alpha-bungarotoxin (αBTX), which labels postsynaptic acetylcholine receptors on muscles. ...
... Vertebrate mutations in the Agrin-LRP4-MuSK pathway, believed to be the predominant pathway for NMJ formation, are characterized by phenotypes of reduced innervation, 10 or complete absence of NMJs. 11,12,14 In the erc1b mutant, we did not observe defects in colocalization, or volume overlap of SV2/αBTX markers, suggesting the mutant phenotype is not likely related to defects in NMJ formation. Instead, the distinct innervation defects in erc1b À/À mutants, compared to those of the Agrin-LRP4-MuSK pathway, suggests that Erc1b is involved in a novel genetic pathway that is required for distribution and organization of NMJs in the lower jaw muscles. ...
... 64 One possible mechanism for NMJ organization involves pre-patterning of acetylcholine receptors on the muscle surface prior to arrival of growing motor axons. 2,11,42 Pre-patterned acetylcholine receptors are observed in zebrafish trunk 1 and mouse skeletal muscles 65 ; however, in our analysis of NMJ development in the craniofacial muscles, we did not observe pre-patterned acetylcholine receptors. This finding may relate to the arrival of trigeminal motor neurons into the first branchial arch before the appearance of muscles. ...
Background: Movement of the lower jaw, a common behavior observed among vertebrates, is required for eating and processing food. This movement is controlled by signals sent from the trigeminal motor nerve through neuromuscular junctions (NMJs) to the masticatory muscles. Dysfunctional jaw movements contribute to craniomandibular disorders, yet the pathophysiology of these disorders is not well understood, as limited studies have been conducted on the molecular mechanisms of jaw movement.
Results: Using erc1b/kimm533 genetic loss of function mutant, we evaluated lower jaw muscle organization and innervation by the cranial motor nerves in developing zebrafish. Using time‐lapse confocal imaging of the erc1b mutant in a transgenic fluorescent reporter line, we found delayed trigeminal nerve growth and disrupted nerve branching architecture during muscle innervation. By automated 3D image analysis of NMJ distribution, we identified an increased number of small, disorganized NMJ clusters in erc1b mutant larvae compared to WT siblings. Using genetic replacement experiments, we determined the Rab GTPase binding domain of Erc1b is required for cranial motor nerve branching, but not NMJ organization or muscle attachment.
Conclusions: We identified Erc1b/ERC1 as a novel component of a genetic pathway contributing to muscle organization, trigeminal nerve outgrowth, and NMJ spatial distribution during development that is required for jaw movement.
... Upon activation, MuSK initiates a downstream signaling cascade to cluster AChRs in apposition to axons, thereby forming neural synapses. Mutants of lrp4, agrn and musk all fail to form NMJs in the mouse and zebrafish trunk, demonstrating their crucial and conserved role in this process Zhang et al., 2008;Jing et al., 2010;Remédio et al., 2016;Gribble et al., 2018). Prior work on the Agrin/Lrp4/MuSK pathway in NMJ development has focused predominantly on trunk muscles. ...
... In the zebrafish trunk, motor axons branch and form synapses distributed along myofibers throughout the myotome. Consistent with previous work (Lefebvre et al., 2007;Jing et al., 2010), at 120 hpf, compared with sibling controls, trunk axial muscle fibers in musk mutants exhibited diffuse α-Btx staining with fewer and smaller AChR clusters ( Fig. 3A,B,D). Similar to trunk axial muscle fibers, appendicular muscle fibers in musk mutants displayed mostly (C) Abductor muscle innervation in the pectoral fin in 120 hpf larvae expressing Tg(mnx1:GFP) to label motoneurons and stained with α-Btx to label AChRs. ...
The receptor tyrosine kinase MuSK, its co-receptor Lrp4 and the Agrin ligand constitute a signaling pathway that is crucial in axial muscle for neuromuscular synapse development, yet whether this pathway functions similarly in appendicular muscle is unclear. Here, using the larval zebrafish pectoral fin, equivalent to tetrapod forelimbs, we show that, similar to axial muscle, developing appendicular muscles form aneural acetylcholine receptor (AChR) clusters prior to innervation. As motor axons arrive, neural AChR clusters form, eventually leading to functional synapses in a MuSK-dependent manner. We find that loss of Agrin or Lrp4 function, which abolishes synaptic AChR clusters in axial muscle, results in enlarged presynaptic nerve regions and progressively expanding appendicular AChR clusters, mimicking the consequences of motoneuron ablation. Moreover, musk depletion in lrp4 mutants partially restores synaptic AChR patterning. Combined, our results provide compelling evidence that, in addition to the canonical pathway in which Agrin/Lrp4 stimulates MuSK activity, Agrin/Lrp4 signaling in appendicular muscle constrains MuSK-dependent neuromuscular synapse organization. Thus, we reveal a previously unappreciated role for Agrin/Lrp4 signaling, thereby highlighting distinct differences between axial and appendicular synapse development.
... We found that the expression of BmSRC1, BmMUSK and BmEGFR was very high in the early embryos of silkworm ( Figure 5, red box). Previous studies in zebra-fish have shown that: the nonreceptor tyrosine kinase SRC participated in egg activation during fertilization [47], MUSK played roles in neuromuscular synapse formation [48], and the EGFR receptor tyrosine kinase family affected the development of neural crest in the early stage of embryos [49]. The EPHR family is crucial for retinotectal system pattern, mesenchymal-to-epithelial transition and extracellular matrix assembly [50][51][52][53], and our results showed BmEPH1 exhibited relatively high expression in the head (Figure 4, red box). ...
... We found that the expression of BmSRC1, BmMUSK and BmEGFR was very high in the early embryos of silkworm ( Figure 5, red box). Previous studies in zebra-fish have shown that: the non-receptor tyrosine kinase SRC participated in egg activation during fertilization [47], MUSK played roles in neuromuscular synapse formation [48], and the EGFR receptor tyrosine kinase family affected the development of neural crest in the early stage of embryos [49]. The EPHR family is crucial for retinotectal system pattern, mesenchymal-to-epithelial transition and extracellular matrix assembly [50][51][52][53], and our results showed BmEPH1 exhibited relatively high expression in the head (Figure 4, red box). ...
The tyrosine kinases (TKs) are important parts of metazoan signaling pathways and play significant roles in cell growth, development, apoptosis and disease. Genome-wide characterization of TKs has been conducted in many metazoans, however, systematic information about this family in Lepidoptera is still lacking. We retrieved 33 TK-encoding genes in silkworm and classified them into 25 subfamilies by sequence analysis, without members in AXL, FRK, PDGFR, STYK1 and TIE subfamilies. Although domain sequences in each subfamily are conserved, TKs in vertebrates tend to be remarkably conserved and stable. Our results of phylogenetic analysis supported the previous conclusion for the second major expansion of TK family. Gene-Ontology (GO) analysis revealed that a higher proportion of BmTKs played roles in binding, catalysis, signal transduction, metabolism, biological regulation and response to stimulus, compared to all silkworm genes annotated in GO. Moreover, the expression profile analysis of BmTKs among multiple tissues and developmental stages demonstrated that many genes exhibited stage-specific and/or sex-related expression during embryogenesis, molting and metamorphosis, and that 8 BmTKs presented tissue-specific high expression. Our study provides systematic description of silkworm tyrosine kinases, and may also provide further insights into metazoan TKs and assist future studies addressing their functions.
... Each of these steps are governed primarily by muscle-derived signals in zebrafish (19,25). This is in-keeping with the fact that it is mainly postsynaptic CMS subtypes in which the effect of b-agonist therapy is beneficial. ...
Inherited defects of the neuromuscular junction (NMJ) comprise an increasingly diverse range of disorders, termed congenital myasthenic syndromes (CMS). Therapies acting on the sympathetic nervous system, including the selective β2 adrenergic agonist salbutamol and the α and β adrenergic agonist ephedrine, have become standard treatment for several types of CMS. However, the mechanism of the therapeutic effect of sympathomimetics in these disorders is not understood. Here, we examined the effect of salbutamol on NMJ development using zebrafish with deficiency of the key postsynaptic proteins Dok-7 and MuSK. Treatment with salbutamol reduced motility defects in zebrafish embryos and larvae. In addition, salbutamol lead to morphological improvement of postsynaptic acetycholine receptor (AChR) clustering and size of synaptic contacts in Dok-7 deficient zebrafish. In MuSK deficient zebrafish, salbutamol treatment reduced motor axon pathfinding defects and partially restored the formation of aneural prepatterned AChRs. In addition, the effects of salbutamol treatment were prevented by pre-treatment with a selective β2 antagonist. Treatment with the cyclic adenosine monophosphate (cAMP) activator forskolin, replicated the effects of salbutamol treatment. These results suggest that sympathomimetics exert a direct effect on neuromuscular synaptogenesis and do so via β2 adrenoceptors and via a cAMP-dependent pathway.
... Dvl interacts with MuSK, and disrupting this interaction in muscles impairs presynaptic development in vivo (63). In zebrafish, Dvl-dependent signaling initiated by Wnt-MuSK interaction in muscle fibers restricts growth cone guidance to the central region of muscle fibers (64,65) and restricts neural crest cells to the center of each somite (66). Many muscle proteins or muscle-derived factors have been implicated in presynaptic differentiation, including embigin (67), fibroblast growth factor (FGF) 7/10/22, collagen IV, glial cell line-derived neurotrophic factor (GDNF), signal regulatory protein-α, laminin β2, and integrin β1. ...
... NMJ deficits in MuSK CRD mutant mice could be rescued by LiCl, an inhibitor of GSK3β, to increase β-catenin, suggesting the involvement of the canonical Wnt signaling (92). In zebrafish, MuSK activation by Wnt is able to initiate Dvl-dependent, noncanonical, or planar cell polarity signaling in muscle fibers that restricts aneural AChR clusters to the central region of muscle fibers (64,65). As observed for agrin activation of MuSK (93), MuSK activation by Wnts stimulates MuSK translocation to recycling endosomes, which is a necessary step for AChR accumulation at the NMJ (90). ...
Synapses, the fundamental unit in neuronal circuits, are critical for learning and memory, perception, thinking, and reaction. The neuromuscular junction (NMJ) is a synapse formed between motoneurons and skeletal muscle fibers that is wrapped by Schwann cells (SCs). It is essential for controlling muscle contraction. NMJ formation requires intimate interactions among motoneurons, muscles, and SCs. Deficits in NMJ formation and maintenance cause neuromuscular disorders, including congenital myasthenic syndrome and myasthenia gravis. NMJ decline occurs in aged animals and may appear before clinical presentation of motoneuron disorders such as amyotrophic lateral sclerosis. We review recent findings in NMJ formation, maintenance, neuromuscular disorders, and aging of the NMJ, focusing on communications among motoneurons, muscles and SCs, and underlying mechanisms. Expected final online publication date for the Annual Review of Physiology Volume 80 is February 10, 2018. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
... 43,44 A range of mutants was identified in large-scale genetic screens, and they display correctly localized AChRs, but these are reduced both in number and size, as well as demonstrating axonal pathfinding defects. [44][45][46][47][48] Despite these morphological defects, motility of these fish is surprisingly normal, and there have been no reports of muscle fiber integrity analysis. These results differ from those found in the Musk V789M/− mice that have a null allele as well as the reported V789M human mutation. ...
Congenital myasthenic syndromes (CMS) are a group of rare disorders that cause fatigable muscle weakness due to defective signal transmission at the neuromuscular junction, a specialized synapse between peripheral motor neurons and their target muscle fibers. There are now over 30 causative genes that have been reported for CMS. Of these, there are 10 that are associated with a limb-girdle pattern of muscle weakness and are thus classed as LG-CMS. Next-generation sequencing and advanced methods of data sharing are likely to uncover further genes that are associated with similar clinical phenotypes, contributing to better diagnosis and effective treatment of LG-CMS patients. This review highlights clinical and pathological hallmarks of LG-CMS in relation to the underlying genetic defects and pathways. Tailored animal and cell models are essential to elucidate the exact function and pathomechanisms at the neuromuscular synapse that underlie LG-CMS. The integration of genomics and proteomics data derived from these models and patients reveals new and often unexpected insights that are relevant beyond the rare genetic disorder of LG-CMS and may extend to the functioning of mammalian synapses in health and disease more generally.
