Wnt signaling promotes AChR aggregation at the neuromuscular synapse in collaboration with agrin

Neuronal Plasticity-Dependent Paradigm and Young Plasma Treatment Prevent Synaptic and Motor Deficit in a Rett Syndrome Mouse Model

Classical Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the MECP2 gene, resulting in a devastating phenotype associated with a lack of gene expression control. Mouse models lacking Mecp2 expression with an RTT-like phenotype have been developed to advance therapeutic alternatives. Environmental enrichment (EE) attenuates RTT symptoms in patients and mouse models. However, the mechanisms underlying the effects of EE on RTT have not been fully elucidated. We housed male hemizygous Mecp2-null (Mecp2-/y) and wild-type mice in specially conditioned cages to enhance sensory, cognitive, social, and motor stimulation. EE attenuated the progression of the RTT phenotype by preserving neuronal cytoarchitecture and neural plasticity markers. Furthermore, EE ameliorated defects in neuromuscular junction organization and restored the motor deficit of Mecp2-/y mice. Treatment with plasma from young WT mice was used to assess whether the increased activity could modify plasma components, mimicking the benefits of EE in Mecp2-/y. Plasma treatment attenuated the RTT phenotype by improving neurological markers, suggesting that peripheral signals of mice with normal motor function have the potential to reactivate dormant neurodevelopment in RTT mice. These findings demonstrate how EE and treatment with young plasma ameliorate RTT-like phenotype in mice, opening new therapeutical approaches for RTT patients.

Targeting Crosstalk of Signaling Pathways among Muscles-Bone-Adipose Tissue: A Promising Therapeutic Approach for Sarcopenia

The aging process is associated with the development of a wide range of degenerative disorders in mammals. These diseases are characterized by a progressive decline in function at multiple levels, including the molecular, cellular, tissue, and organismal. Furthermore, it is responsible for various healthcare costs in developing and developed countries. Sarcopenia is the deterioration in the quality and functionality of muscles, which is extremely concerning as it manages many functions in the human body. This article reviews the molecular crosstalk involved in sarcopenia and the specific roles of many mediator molecules in establishing cross-talk between muscles, bone, and fatty tissues, eventually leading to sarcopenia. Besides, the involvement of various etiological factors, such as neurology, endocrinology, lifestyle, etc., makes it exceedingly difficult for clinicians to develop a coherent hypothesis that may lead to the well-organized management system required to battle this debilitating disease. The several hallmarks contributing to the progression of the disease is a vital question that needs to be addressed to ensure an efficient treatment for sarcopenia patients. Also, the intricate molecular mechanism involved in developing this disease requires more studies. The direct relationship of cellular senescence with aging is one of the pivotal issues contributing to disease pathophysiology. Some patented treatment strategies have been discussed, including drugs undergoing clinical trials and emerging options like miRNA and protein-enclosed extracellular vesicles. A clear understanding of the secretome, including the signaling pathways involved between muscles, bone, and fatty tissues, is extremely beneficial for developing novel therapeutics for curing sarcopenia.

Aberrant evoked calcium signaling and nAChR cluster morphology in a SOD1 D90A hiPSC-derived neuromuscular model

Familial amyotrophic lateral sclerosis (ALS) is a progressive neuromuscular disorder that is due to mutations in one of several target genes, including SOD1. So far, clinical records, rodent studies, and in vitro models have yielded arguments for either a primary motor neuron disease, or a pleiotropic pathogenesis of ALS. While mouse models lack the human origin, in vitro models using human induced pluripotent stem cells (hiPSC) have been recently developed for addressing ALS pathogenesis. In spite of improvements regarding the generation of muscle cells from hiPSC, the degree of maturation of muscle cells resulting from these protocols has remained limited. To fill these shortcomings, we here present a new protocol for an enhanced myotube differentiation from hiPSC with the option of further maturation upon coculture with hiPSC-derived motor neurons. The described model is the first to yield a combination of key myogenic maturation features that are consistent sarcomeric organization in association with complex nAChR clusters in myotubes derived from control hiPSC. In this model, myotubes derived from hiPSC carrying the SOD1 D90A mutation had reduced expression of myogenic markers, lack of sarcomeres, morphologically different nAChR clusters, and an altered nAChR-dependent Ca2+ response compared to control myotubes. Notably, trophic support provided by control hiPSC-derived motor neurons reduced nAChR cluster differences between control and SOD1 D90A myotubes. In summary, a novel hiPSC-derived neuromuscular model yields evidence for both muscle-intrinsic and nerve-dependent aspects of neuromuscular dysfunction in SOD1-based ALS.

The impact of canonical Wnt transcriptional repressors TLE3 and TLE4 on postsynaptic transcription at the neuromuscular junction

Here, we investigated the role of the canonical Wnt signaling pathway transcriptional regulators at the neuromuscular junction. Upon applying a denervation paradigm, the transcription levels of Ctnnb1, Tcf7l1, Tle1, Tle2, Tle3, and Tle4 were significantly downregulated. A significant decrease in canonical Wnt signaling activity was observed using the denervation paradigm in Axin2-lacZ reporter mice. Alterations in the transcriptional profile of the myogenic lineage in response to agrin (AGRN) suggested that TLE3 and TLE4, family members of groucho transducin-like enhancer of split 3 (TLE3), transcriptional repressors known to antagonize T cell factor/lymphoid enhancer factor (TCF)-mediated target gene activation, could be important regulators of canonical Wnt signaling activity at the postsynapse. Knockouts of these genes using CRISPR/Cas9 gene editing in primary skeletal muscle stem cells, called satellite cells, led to decreased AGRN-dependent acetylcholine receptor (CHRN) clustering and reduced synaptic gene transcription upon differentiation of these cells. Overall, our findings demonstrate that TLE3 and TLE4 participate in diminishing canonical Wnt signaling activity, supporting transcription of synaptic genes and CHRN clustering at the neuromuscular junction.

LRP4-related signalling pathways and their regulatory role in neurological diseases

The mechanism of action of low-density lipoprotein receptor related protein 4 (LRP4) is mediated largely via the Agrin-LRP4-MuSK signalling pathway in the nervous system. LRP4 contributes to the development of synapses in the peripheral nervous system (PNS). It interacts with signalling molecules such as the amyloid beta-protein precursor (APP) and the wingless type protein (Wnt). Its mechanisms of action are complex and mediated via interaction between the pre-synaptic motor neuron and post-synaptic muscle cell in the PNS, which enhances the development of the neuromuscular junction (NMJ). LRP4 may function differently in the central nervous system (CNS) than in the PNS, where it regulates ATP and glutamate release via astrocytes. It mayaffect the growth and development of the CNS by controlling the energy metabolism. LRP4 interacts with Agrin to maintain dendrite growth and density in the CNS. The goal of this article is to review the current studies involving relevant LRP4 signaling pathways in the nervous system. The review also discusses the clinical and etiological roles of LRP4 in neurological illnesses, such as myasthenia gravis, Alzheimer's disease and epilepsy. In this review, we provide a theoretical foundation for the pathogenesis and therapeutic application of LRP4 in neurologic diseases.

IgG1-3 MuSK Antibodies Inhibit AChR Cluster Formation, Restored by SHP2 Inhibitor, Despite Normal MuSK, DOK7, or AChR Subunit Phosphorylation

BACKGROUND AND OBJECTIVES

Up to 50% of patients with myasthenia gravis (MG) without acetylcholine receptor antibodies (AChR-Abs) have antibodies to muscle-specific kinase (MuSK). Most MuSK antibodies (MuSK-Abs) are IgG4 and inhibit agrin-induced MuSK phosphorylation, leading to impaired clustering of AChRs at the developing or mature neuromuscular junction. However, IgG1-3 MuSK-Abs also exist in MuSK-MG patients, and their potential mechanisms have not been explored fully.

METHODS

C2C12 myotubes were exposed to MuSK-MG plasma IgG1-3 or IgG4, with or without purified agrin. MuSK, Downstream of Kinase 7 (DOK7), and βAChR were immunoprecipitated and their phosphorylation levels identified by immunoblotting. Agrin and agrin-independent AChR clusters were measured by immunofluorescence and AChR numbers by binding of 125I-α-bungarotoxin. Transcriptomic analysis was performed on treated myotubes.

RESULTS

IgG1-3 MuSK-Abs impaired AChR clustering without inhibiting agrin-induced MuSK phosphorylation. Moreover, the well-established pathway initiated by MuSK through DOK7, resulting in βAChR phosphorylation, was not impaired by MuSK-IgG1-3 and was agrin-independent. Nevertheless, the AChR clusters did not form, and both the number of AChR microclusters that precede full cluster formation and the myotube surface AChRs were reduced. Transcriptomic analysis did not throw light on the pathways involved. However, the SHP2 inhibitor, NSC-87877, increased the number of microclusters and led to fully formed AChR clusters.

DISCUSSION

MuSK-IgG1-3 is pathogenic but seems to act through a noncanonical pathway. Further studies should throw light on the mechanisms involved at the neuromuscular junction.

WNT-β Catenin Signaling as a Potential Therapeutic Target for Neurodegenerative Diseases: Current Status and Future Perspective

Wnt/β-catenin (WβC) signaling pathway is an important signaling pathway for the maintenance of cellular homeostasis from the embryonic developmental stages to adulthood. The canonical pathway of WβC signaling is essential for neurogenesis, cell proliferation, and neurogenesis, whereas the noncanonical pathway (WNT/Ca²⁺ and WNT/PCP) is responsible for cell polarity, calcium maintenance, and cell migration. Abnormal regulation of WβC signaling is involved in the pathogenesis of several neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), and spinal muscular atrophy (SMA). Hence, the alteration of WβC signaling is considered a potential therapeutic target for the treatment of neurodegenerative disease. In the present review, we have used the bibliographical information from PubMed, Google Scholar, and Scopus to address the current prospects of WβC signaling role in the abovementioned neurodegenerative diseases.

Landscape analysis of m6A modification regulators related biological functions and immune characteristics in myasthenia gravis

BACKGROUND

N6-methyladenosine (m6A) modification has been recognized to play fundamental roles in the development of autoimmune diseases. However, the implication of m6A modification in myasthenia gravis (MG) remains largely unknown. Thus, we aimed to systematically explore the potential functions and related immune characteristics of m6A regulators in MG.

METHODS

The GSE85452 dataset with MG and healthy samples was downloaded from Gene Expression Omnibus (GEO) database. m6A modification regulators were manually curated. The targets of m6A regulators were obtained from m6A2Target database. The differential expressed m6A regulators in GSE85452 dataset were identified by "limma" package and were validated by RT-PCR. Function enrichment analysis of dysregulated m6A regulators was performed using "clusterProfiler" package. Correlation analysis was applied for analyzing the relationships between m6A regulators and immune characteristics. Unsupervised clustering analysis was used to identify distinct m6A modification subtypes. The differences between subtypes were analyzed, including the expression level of all genes and the enrichment degree of immune characteristics. Weighted gene co-expression network analysis (WGCNA) was conducted to obtain modules associated with m6A modification subtypes.

RESULTS

We found that CBLL1, RBM15 and YTHDF1 were upregulated in MG samples of GSE85452 dataset, and the results were verified by RT-PCR in blood samples from19 MG patients and 19 controls. The targeted genes common modified by CBLL1, RBM15, and YTHDF1 were mainly enriched in histone modification and Wnt signaling pathway. Correlation analysis showed that three dysregulated m6A regulators were closely associated with immune characteristics. Among them, RBM15 possessed the strongest correlation with immune characteristics, including CD56dim natural killer cell (r = 0.77, P = 0.0023), T follicular helper cell (r = - 0.86, P = 0.0002), Interferon Receptor (r = 0.78, P = 0.0017), and HLA-DOA (r = 0.64, P = 0.0200). Further two distinct m6A modification patterns mediated by three dysregulated m6A regulators was identified. Bioinformatics analysis found that there were 3029 differentially expressed genes and different immune characteristics between two m6A modification patterns. Finally, WGCNA analysis obtained a total of 12 modules and yellow module was the most positively correlated to subtype-2.

CONCLUSION

Our findings suggested that m6A RNA modification had an important effect on immunity molecular mechanism of MG and provided a new perspective into understanding the pathogenesis of MG.

FUS-ALS hiPSC-derived astrocytes impair human motor units through both gain-of-toxicity and loss-of-support mechanisms

BACKGROUND

Astrocytes play a crucial, yet not fully elucidated role in the selective motor neuron pathology in amyotrophic lateral sclerosis (ALS). Among other responsibilities, astrocytes provide important neuronal homeostatic support, however this function is highly compromised in ALS. The establishment of fully human coculture systems can be used to further study the underlying mechanisms of the dysfunctional intercellular interplay, and has the potential to provide a platform for revealing novel therapeutic entry points.

METHODS

In this study, we characterised human induced pluripotent stem cell (hiPSC)-derived astrocytes from FUS-ALS patients, and incorporated these cells into a human motor unit microfluidics model to investigate the astrocytic effect on hiPSC-derived motor neuron network and functional neuromuscular junctions (NMJs) using immunocytochemistry and live-cell recordings. FUS-ALS cocultures were systematically compared to their CRISPR-Cas9 gene-edited isogenic control systems.

RESULTS

We observed a dysregulation of astrocyte homeostasis, which resulted in a FUS-ALS-mediated increase in reactivity and secretion of inflammatory cytokines. Upon coculture with motor neurons and myotubes, we detected a cytotoxic effect on motor neuron-neurite outgrowth, NMJ formation and functionality, which was improved or fully rescued by isogenic control astrocytes. We demonstrate that ALS astrocytes have both a gain-of-toxicity and loss-of-support function involving the WNT/β-catenin pathway, ultimately contributing to the disruption of motor neuron homeostasis, intercellular networks and NMJs.

CONCLUSIONS

Our findings shine light on a complex, yet highly important role of astrocytes in ALS, and provides further insight in to their pathological mechanisms.

In Adult Skeletal Muscles, the Co-Receptors of Canonical Wnt Signaling, Lrp5 and Lrp6, Determine the Distribution and Size of Fiber Types, and Structure and Function of Neuromuscular Junctions

Canonical Wnt signaling is involved in skeletal muscle cell biology. The exact way in which this pathway exerts its contribution to myogenesis or neuromuscular junctions (NMJ) is a matter of debate. Next to the common co-receptors of canonical Wnt signaling, Lrp5 and Lrp6, the receptor tyrosine kinase MuSK was reported to bind at NMJs WNT glycoproteins by its extracellular cysteine-rich domain. Previously, we reported canonical Wnt signaling being active in fast muscle fiber types. Here, we used conditional Lrp5 or Lrp6 knockout mice to investigate the role of these receptors in muscle cells. Conditional double knockout mice died around E13 likely due to ectopic expression of the Cre recombinase. Phenotypes of single conditional knockout mice point to a very divergent role for the two receptors. First, muscle fiber type distribution and size were changed. Second, canonical Wnt signaling reporter mice suggested less signaling activity in the absence of Lrps. Third, expression of several myogenic marker genes was changed. Fourth, NMJs were of fragmented phenotype. Fifth, recordings revealed impaired neuromuscular transmission. In sum, our data show fundamental differences in absence of each of the Lrp co-receptors and suggest a differentiated view of canonical Wnt signaling pathway involvement in adult skeletal muscle cells.

Deconstruction of Neurotrypsin Reveals a Multi-factorially Regulated Activity Affecting Myotube Formation and Neuronal Excitability

Neurotrypsin (NT) is a highly specific nervous system multi-domain serine protease best known for its selective processing of the potent synaptic organizer agrin. Its enzymatic activity is thought to influence processes of synaptic plasticity, with its deregulation causing accelerated neuromuscular junction (NMJ) degeneration or contributing to forms of mental retardation. These biological effects are likely to stem from NT-based regulation of agrin signaling. However, dissecting the exact biological implications of NT-agrin interplay is difficult, due to the scarce molecular detail regarding NT activity and NT-agrin interactions. We developed a strategy to reliably produce and purify a catalytically competent engineered variant of NT called "NT-mini" and a library of C-terminal agrin fragments, with which we performed a thorough biochemical and biophysical characterization of NT enzyme functionality. We studied the regulatory effects of calcium ions and heparin, identified NT's heparin-binding domain, and discovered how zinc ions induce modulation of enzymatic activity. Additionally, we investigated myotube differentiation and hippocampal neuron excitability, evidencing a dose-dependent increase in neuronal activity alongside a negative impact on myoblast fusion when using the active NT enzyme. Collectively, our results provide in vitro and cellular foundations to unravel the molecular underpinnings and biological significance of NT-agrin interactions.

Maturation of a postsynaptic domain: Role of small Rho GTPases in organising nicotinic acetylcholine receptor aggregates at the vertebrate neuromuscular junction

The neuromuscular junction (NMJ) is the peripheral synapse formed between a motor axon and a skeletal muscle fibre that allows muscle contraction and the coordinated movement in many species. A main hallmark of the mature NMJ is the assembly of nicotinic acetylcholine receptor (nAChR) aggregates in the muscle postsynaptic domain, that distributes in perfect apposition to presynaptic motor terminals. To assemble its unique functional architecture, initial embryonic NMJs undergo an early postnatal maturation process characterised by the transformation of homogenous nAChR-containing plaques to elaborate and branched pretzel-like structures. In spite of a detailed morphological characterisation, the molecular mechanisms controlling the intracellular scaffolding that organises a postsynaptic domain at the mature NMJ have not been fully elucidated. In this review, we integrate evidence of key processes and molecules that have shed light on our current understanding of the NMJ maturation process. On the one hand, we consider in vitro studies revealing the potential role of podosome-like structures to define discrete low nAChR-containing regions to consolidate a plaque-to-pretzel transition at the NMJ. On the other hand, we focus on in vitro and in vivo evidence demonstrating that members of the Ras homologous (Rho) protein family of small GTPases (small Rho GTPases) play indispensable roles on NMJ maturation by regulating the stability of nAChR aggregates. We combine this evidence to propose that small Rho GTPases are key players in the assembly of podosome-like structures that drive the postsynaptic maturation of vertebrate NMJs.

The cell polarity protein Vangl2 in the muscle shapes the neuromuscular synapse by binding to and regulating the tyrosine kinase MuSK

The development of the neuromuscular junction (NMJ) requires dynamic trans-synaptic coordination orchestrated by secreted factors, including Wnt family morphogens. To investigate how these synaptic cues in NMJ development are transduced, particularly in the regulation of acetylcholine receptor (AChR) accumulation in the postsynaptic membrane, we explored the function of Van Gogh-like protein 2 (Vangl2), a core component of Wnt planar cell polarity signaling. We found that conditional, muscle-specific ablation of Vangl2 in mice reproduced the NMJ differentiation defects seen in mice with global Vangl2 deletion. These alterations persisted into adulthood and led to NMJ disassembly, impaired neurotransmission, and deficits in motor function. Vangl2 and the muscle-specific receptor tyrosine kinase MuSK were functionally associated in Wnt signaling in the muscle. Vangl2 bound to and promoted the signaling activity of MuSK in response to Wnt11. The loss of Vangl2 impaired RhoA activation in cultured mouse myotubes and caused dispersed, rather than clustered, organization of AChRs at the postsynaptic or muscle cell side of NMJs in vivo. Our results identify Vangl2 as a key player of the core complex of molecules shaping neuromuscular synapses and thus shed light on the molecular mechanisms underlying NMJ assembly.

Transport and Secretion of the Wnt3 Ligand by Motor Neuron-like Cells and Developing Motor Neurons

The vertebrate neuromuscular junction (NMJ) is formed by a presynaptic motor nerve terminal and a postsynaptic muscle specialization. Cumulative evidence reveals that Wnt ligands secreted by the nerve terminal control crucial steps of NMJ synaptogenesis. For instance, the Wnt3 ligand is expressed by motor neurons at the time of NMJ formation and induces postsynaptic differentiation in recently formed muscle fibers. However, the behavior of presynaptic-derived Wnt ligands at the vertebrate NMJ has not been deeply analyzed. Here, we conducted overexpression experiments to study the expression, distribution, secretion, and function of Wnt3 by transfection of the motor neuron-like NSC-34 cell line and by in ovo electroporation of chick motor neurons. Our findings reveal that Wnt3 is transported along motor axons in vivo following a vesicular-like pattern and reaches the NMJ area. In vitro, we found that endogenous Wnt3 expression increases as the differentiation of NSC-34 cells proceeds. Although NSC-34 cells overexpressing Wnt3 do not modify their morphological differentiation towards a neuronal phenotype, they effectively induce acetylcholine receptor clustering on co-cultured myotubes. These findings support the notion that presynaptic Wnt3 is transported and secreted by motor neurons to induce postsynaptic differentiation in nascent NMJs.

Agrin/Lrp4 signal constrains MuSK-dependent neuromuscular synapse development in appendicular muscle

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.

Metabolic Contributions of Wnt Signaling: More Than Controlling Flight

The canonical Wnt signaling pathway is ubiquitous throughout the body and influences a diverse array of physiological processes. Following the initial discovery of the Wnt signaling pathway during wing development in Drosophila melanogaster, it is now widely appreciated that active Wnt signaling in mammals is necessary for the development and growth of various tissues involved in whole-body metabolism, such as brain, liver, pancreas, muscle, and adipose. Moreover, elegant gain- and loss-of-function studies have dissected the tissue-specific roles of various downstream effector molecules in the regulation of energy homeostasis. This review attempts to highlight and summarize the contributions of the Wnt signaling pathway and its downstream effectors on whole-body metabolism and their influence on the development of metabolic diseases, such as diabetes and obesity. A better understanding of the Wnt signaling pathway in these tissues may aid in guiding the development of future therapeutics to treat metabolic diseases.

Murine muscle stem cell response to perturbations of the neuromuscular junction are attenuated with aging

During aging and neuromuscular diseases, there is a progressive loss of skeletal muscle volume and function impacting mobility and quality of life. Muscle loss is often associated with denervation and a loss of resident muscle stem cells (satellite cells or MuSCs); however, the relationship between MuSCs and innervation has not been established. Herein, we administered severe neuromuscular trauma to a transgenic murine model that permits MuSC lineage tracing. We show that a subset of MuSCs specifically engraft in a position proximal to the neuromuscular junction (NMJ), the synapse between myofibers and motor neurons, in healthy young adult muscles. In aging and in a mouse model of neuromuscular degeneration (Cu/Zn superoxide dismutase knockout - Sod1-/-), this localized engraftment behavior was reduced. Genetic rescue of motor neurons in Sod1-/- mice reestablished integrity of the NMJ in a manner akin to young muscle and partially restored MuSC ability to engraft into positions proximal to the NMJ. Using single cell RNA-sequencing of MuSCs isolated from aged muscle, we demonstrate that a subset of MuSCs are molecularly distinguishable from MuSCs responding to myofiber injury and share similarity to synaptic myonuclei. Collectively, these data reveal unique features of MuSCs that respond to synaptic perturbations caused by aging and other stressors.

Synapse-specific Lrp4 mRNA enrichment requires Lrp4/MuSK signaling, muscle activity and Wnt non-canonical pathway

Background

The neuromuscular junction (NMJ) is a peripheral synapse critical to muscle contraction. Like acetylcholine receptors (AChRs), many essential proteins of NMJ are extremely concentrated at the postjunctional membrane. However, the mechanisms of synapse-specific concentration are not well understood; furthermore, it is unclear whether signaling molecules critical to NMJ formation and maintenance are also locally transcribed.

Results

We studied the β-gal activity encoded by a lacZ cassette driven by the promoter of the Lrp4 gene. As reported for Lrp4 mRNA, β-gal was in the central region in embryonic muscles and at the NMJ after its formation. However, β-gal was no longer in the central areas of muscle fibers in Lrp4 or MuSK mutant mice, indicating a requirement of Lrp4/MuSK signaling. This phenotype could be rescued by transgenic expression of LRP4 with a transmembrane domain but not soluble ECD in Lrp4 mutant mice. β-gal and AChR clusters were distributed in a broader region in lacZ/ECD than that of heterozygous lacZ/+ mice, indicating an important role of the transmembrane domain in Lrp4 signaling. Synaptic β-gal activity became diffused after denervation or treatment with µ-conotoxin, despite its mRNA was increased, indicating synaptic Lrp4 mRNA enrichment requires muscle activity. β-gal was also diffused in aged mice but became re-concentrated after muscle stimulation. Finally, Lrp4 mRNA was increased in C2C12 myotubes by Wnt ligands in a manner that could be inhibited by RKI-1447, an inhibitor of ROCK in Wnt non-canonical signaling. Injecting RKI-1447 into muscles of adult mice diminished Lrp4 synaptic expression.

Conclusions

This study demonstrates that synapse-specific enrichment of Lrp4 mRNA requires a coordinated interaction between Lrp4/MuSK signaling, muscle activity, and Wnt non-canonical signaling. Thus, the study provides a new mechanism for Lrp4 mRNA enrichment. It also provides a potential target for the treatment of NMJ aging and other NMJ-related diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00619-z.

Potential Roles of the WNT Signaling Pathway in Amyotrophic Lateral Sclerosis

The WNT signaling pathway plays an important role in the physiological and pathophysiological processes of the central nervous system and the neurodegenerative disease amyotrophic lateral sclerosis (ALS). We reviewed the literature pertinent to WNT/β–catenin signaling in ALS from cellular studies, animal models, and human clinical trials. WNT, WNT receptors, and other components of the WNT signaling pathway are expressed in both ALS patients and transgenic mice, and are involved in the pathogenesis of ALS. Studies have shown that abnormal activation of the WNT/β–catenin signaling pathway is related to neuronal degeneration and glial cell proliferation. WNT/Ca²⁺ signaling is associated with the pro–inflammatory phenotype of microglia; data on the muscle skeletal receptor Tyr kinase receptor in superoxide dismutase–1–G93A mice indicate that gene therapy is necessary for successful treatment of ALS. The varying profiles of lipoprotein receptor–related protein 4 antibodies in different ethnic groups suggest that individual treatment and multifactorial personalized approaches may be necessary for effective ALS therapy. In conclusion, the WNT signaling pathway is important to the ALS disease process, making it a likely therapeutic target.

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.

Secreted Signaling Molecules at the Neuromuscular Junction in Physiology and Pathology

Signal transduction at the neuromuscular junction (NMJ) is affected in many human diseases, including congenital myasthenic syndromes (CMS), myasthenia gravis, Lambert–Eaton myasthenic syndrome, Isaacs’ syndrome, Schwartz–Jampel syndrome, Fukuyama-type congenital muscular dystrophy, amyotrophic lateral sclerosis, and sarcopenia. The NMJ is a prototypic cholinergic synapse between the motor neuron and the skeletal muscle. Synaptogenesis of the NMJ has been extensively studied, which has also been extrapolated to further understand synapse formation in the central nervous system. Studies of genetically engineered mice have disclosed crucial roles of secreted molecules in the development and maintenance of the NMJ. In this review, we focus on the secreted signaling molecules which regulate the clustering of acetylcholine receptors (AChRs) at the NMJ. We first discuss the signaling pathway comprised of neural agrin and its receptors, low-density lipoprotein receptor-related protein 4 (Lrp4) and muscle-specific receptor tyrosine kinase (MuSK). This pathway drives the clustering of acetylcholine receptors (AChRs) to ensure efficient signal transduction at the NMJ. We also discuss three secreted molecules (Rspo2, Fgf18, and connective tissue growth factor (Ctgf)) that we recently identified in the Wnt/β-catenin and fibroblast growth factors (FGF) signaling pathways. The three secreted molecules facilitate the clustering of AChRs by enhancing the agrin-Lrp4-MuSK signaling pathway.

Regulation of Gene expression at the neuromuscular Junction

Gene expression in skeletal muscle is profoundly changed upon innervation. 50 years of research on the neuromuscular system have greatly increased our understanding of the mechanisms underlying these changes. By controlling the expression and the activity of key transcription factors, nerve-evoked electrical activity in the muscle fiber positively and negatively regulates the expression of hundreds of genes. Innervation also compartmentalizes gene expression into synaptic and extra-synaptic regions of muscle fibers. In addition, electrically-evoked, release of several factors (e.g. Agrin, Neuregulin, Wnt ligands) induce the clustering of synaptic proteins and of a few muscle nuclei. The sub-synaptic nuclei acquire a particular chromatin organization and develop a specific gene expression program dedicated to building and maintaining a functional neuromuscular synapse. Deciphering synapse-specific, transcriptional regulation started with the identification of the N-box, a six base pair element present in the promoters of the acetylcholine δ and ε subunits. Most genes with synapse-specific expression turned out to contain at least one N-box in their promoters. The N-box is a response element for the synaptic signals Agrin and Neuregulins as well as a binding site for transcription factors of the Ets family. The Ets transcription factors GABP and Erm are implicated in the activation of post-synaptic genes via the N-box. In muscle fibers, Erm expression is restricted to the NMJ whereas GABP is expressed in all muscle nuclei but phosphorylated and activated by the JNK and ERK signaling pathways in response to Agrin and Neuregulins. Post-synaptic gene expression also correlates with chromatin modifications at the genomic level as evidenced by the strong enrichment of decondensed chromatin and acetylated histones in sub-synaptic nuclei. Here we discuss these transcriptional pathways for synaptic specialization at NMJs.

Myasthenia Gravis: From the Viewpoint of Pathogenicity Focusing on Acetylcholine Receptor Clustering, Trans-Synaptic Homeostasis and Synaptic Stability

Myasthenia gravis (MG) is a disease of the postsynaptic neuromuscular junction (NMJ) where nicotinic acetylcholine (ACh) receptors (AChRs) are targeted by autoantibodies. Search for other pathogenic antigens has detected the antibodies against muscle-specific tyrosine kinase (MuSK) and low-density lipoprotein-related protein 4 (Lrp4), both causing pre- and post-synaptic impairments. Agrin is also suspected as a fourth pathogen. In a complex NMJ organization centering on MuSK: (1) the Wnt non-canonical pathway through the Wnt-Lrp4-MuSK cysteine-rich domain (CRD)-Dishevelled (Dvl, scaffold protein) signaling acts to form AChR prepatterning with axonal guidance; (2) the neural agrin-Lrp4-MuSK (Ig1/2 domains) signaling acts to form rapsyn-anchored AChR clusters at the innervated stage of muscle; (3) adaptor protein Dok-7 acts on MuSK activation for AChR clustering from “inside” and also on cytoskeleton to stabilize AChR clusters by the downstream effector Sorbs1/2; (4) the trans-synaptic retrograde signaling contributes to the presynaptic organization via: (i) Wnt-MuSK CRD-Dvl-β catenin-Slit 2 pathway; (ii) Lrp4; and (iii) laminins. The presynaptic Ca²⁺ homeostasis conditioning ACh release is modified by autoreceptors such as M1-type muscarinic AChR and A2A adenosine receptors. The post-synaptic structure is stabilized by: (i) laminin-network including the muscle-derived agrin; (ii) the extracellular matrix proteins (including collagen Q/perlecan and biglycan which link to MuSK Ig1 domain and CRD); and (iii) the dystrophin-associated glycoprotein complex. The study on MuSK ectodomains (Ig1/2 domains and CRD) recognized by antibodies suggested that the MuSK antibodies were pathologically heterogeneous due to their binding to multiple functional domains. Focussing one of the matrix proteins, biglycan which functions in the manner similar to collagen Q, our antibody assay showed the negative result in MG patients. However, the synaptic stability may be impaired by antibodies against MuSK ectodomains because of the linkage of biglycan with MuSK Ig1 domain and CRD. The pathogenic diversity of MG is discussed based on NMJ signaling molecules.

Expression of ALS-linked SOD1 Mutation in Motoneurons or Myotubes Induces Differential Effects on Neuromuscular Function In vitro

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that selectively affects upper and lower motoneurons. Dismantlement of the neuromuscular junction (NMJ) is an early pathological hallmark of the disease whose cellular origin remains still debated. We developed an in vitro NMJ model to investigate the differential contribution of motoneurons and muscle cells expressing ALS-causing mutation in the superoxide dismutase 1 (SOD1) to neuromuscular dysfunction. The primary co-culture system allows the formation of functional NMJs and fosters the expression of the ALS-sensitive fast fatigable type II-b myosin heavy chain (MHC) isoform. Expression of SOD1^G93A in myotubes does not prevent the formation of a functional NMJ but leads to decreased contraction frequency and lowers the slow type I MHC isoform transcript levels. Expression of SOD1^G93A in both motoneurons and myotubes or in motoneurons alone however alters the formation of a functional NMJ. Our results strongly suggest that motoneurons are a major factor involved in the process of NMJ dismantlement in an experimental model of ALS.

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.

Dishevelled-1 regulated apoptosis through NF-κB in cerebral ischemia/reperfusion injury in rats

Dishevelled-1(DVL-1) has been reported associated with the regulation of cell polarity and neuronal function. However, the effect of DVL-1 in cerebral ischemia-reperfusion injury of rats remains poorly understood. In this study, we give evidence that the level of DVL-1 is increased after a middle cerebral artery occlusion/reperfusion model (MCAO) in rats, with a peak at 12 h. On the side, knockdown of DVL-1 may relieve I/R damage and restrain apoptosis after MCAO model in rats. In the part of mechanism, DVL-1 could regulate apoptosis through NF-κB. These results suggest that DVL-1 may be a potential target in I/R injury in rats.

Dissecting the Extracellular Complexity of Neuromuscular Junction Organizers

Synapse formation is a very elaborate process dependent upon accurate coordination of pre and post-synaptic specialization, requiring multiple steps and a variety of receptors and signaling molecules. Due to its relative structural simplicity and the ease in manipulation and observation, the neuromuscular synapse or neuromuscular junction (NMJ)-the connection between motor neurons and skeletal muscle-represents the archetype junction system for studying synapse formation and conservation. This junction is essential for survival, as it controls our ability to move and breath. NMJ formation requires coordinated interactions between motor neurons and muscle fibers, which ultimately result in the formation of a highly specialized post-synaptic architecture and a highly differentiated nerve terminal. Furthermore, to ensure a fast and reliable synaptic transmission following neurotransmitter release, ligand-gated channels (acetylcholine receptors, AChRs) are clustered on the post-synaptic muscle cell at high concentrations in sites opposite the presynaptic active zone, supporting a direct role for nerves in the organization of the post-synaptic membrane architecture. This organized clustering process, essential for NMJ formation and for life, relies on key signaling molecules and receptors and is regulated by soluble extracellular molecules localized within the synaptic cleft. Notably, several mutations as well as auto-antibodies against components of these signaling complexes have been related to neuromuscular disorders. The recent years have witnessed strong progress in the understanding of molecular identities, architectures, and functions of NMJ macromolecules. Among these, prominent roles have been proposed for neural variants of the proteoglycan agrin, its receptor at NMJs composed of the lipoprotein receptor-related protein 4 (LRP4) and the muscle-specific kinase (MuSK), as well as the regulatory soluble synapse-specific protease Neurotrypsin. In this review we summarize the current state of the art regarding molecular structures and (agrin-dependent) canonical, as well as (agrin-independent) non-canonical, MuSK signaling mechanisms that underscore the formation of neuromuscular junctions, with the aim of providing a broad perspective to further stimulate molecular, cellular and tissue biology investigations on this fundamental intercellular contact.

Functional link between plasma membrane spatiotemporal dynamics, cancer biology, and dietary membrane-altering agents

The cell plasma membrane serves as a nexus integrating extra- and intracellular components, which together enable many of the fundamental cellular signaling processes that sustain life. In order to perform this key function, plasma membrane components assemble into well-defined domains exhibiting distinct biochemical and biophysical properties that modulate various signaling events. Dysregulation of these highly dynamic membrane domains can promote oncogenic signaling. Recently, it has been demonstrated that select membrane-targeted dietary bioactives (MTDBs) have the ability to remodel plasma membrane domains and subsequently reduce cancer risk. In this review, we focus on the importance of plasma membrane domain structural and signaling functionalities as well as how loss of membrane homeostasis can drive aberrant signaling. Additionally, we discuss the intricacies associated with the investigation of these membrane domain features and their associations with cancer biology. Lastly, we describe the current literature focusing on MTDBs, including mechanisms of chemoprevention and therapeutics in order to establish a functional link between these membrane-altering biomolecules, tuning of plasma membrane hierarchal organization, and their implications in cancer prevention.

Wnt Secretion Is Regulated by the Tetraspan Protein HIC-1 through Its Interaction with Neurabin/NAB-1

The aberrant regulation of Wnt secretion is implicated in various neurological diseases. However, the mechanisms of Wnt release are still largely unknown. Here we describe the role of a C. elegans tetraspan protein, HIC-1, in maintaining normal Wnt release. We show that HIC-1 is expressed in cholinergic synapses and that mutants in hic-1 show increased levels of the acetylcholine receptor AChR/ACR-16. Our results suggest that HIC-1 maintains normal AChR/ACR-16 levels by regulating normal Wnt release from presynaptic neurons, as hic-1 mutants show an increase in secreted Wnt from cholinergic neurons. We further show that HIC-1 affects Wnt secretion by modulating the actin cytoskeleton through its interaction with the actin-binding protein NAB-1. In summary, we describe a protein, HIC-1, that functions as a neuromodulator by affecting postsynaptic AChR/ACR-16 levels by regulating presynaptic Wnt release from cholinergic motor neurons.

Wnt signalling in the development of axon, dendrites and synapses

Wnts are a highly conserved family of secreted glycoproteins that play essential roles in the morphogenesis and body patterning during the development of metazoan species. In recent years, mounting evidence has revealed important functions of Wnt signalling in diverse aspects of neural development, including neuronal polarization, guidance and branching of the axon and dendrites, as well as synapse formation and its structural remodelling. In contrast to Wnt signalling in cell proliferation and differentiation, which mostly acts through β-catenin-dependent pathways, Wnts engage a diverse array of non-transcriptional cascades in neuronal development, such as the planar cell polarity, cytoskeletal or calcium signalling pathways. In this review, we summarize recent advances in the mechanisms of Wnt signalling in the development of axon, dendrite and synapse formation.

Balanced Rac1 activity controls formation and maintenance of neuromuscular acetylcholine receptor clusters

Rac1, an important Rho GTPase that regulates the actin cytoskeleton, has long been suggested to participate in acetylcholine receptor (AChR) clustering at the postsynaptic neuromuscular junction. However, how Rac1 is regulated and how it influences AChR clusters have remained unexplored. This study shows that breaking the balance of Rac1 regulation, by either increasing or decreasing its activity, led to impaired formation and maintenance of AChR clusters. By manipulating Rac1 activity at different stages of AChR clustering in cultured myotubes, we show that Rac1 activation was required for the initial formation of AChR clusters, but its persistent activation led to AChR destabilization, and uncontrolled hyperactivation of Rac1 even caused excessive myotube fusion. Both AChR dispersal and myotube fusion induced by Rac1 were dependent on its downstream effector Pak1. Two Rac1 GAPs and six Rac1 GEFs were screened and found to be important for normal AChR clustering. This study reveals that, although general Rac1 activity remains at low levels during terminal differentiation of myotubes and AChR cluster maintenance, tightly regulated Rac1 activity controls normal AChR clustering.

The Neuromuscular Junction and Wide Heterogeneity of Congenital Myasthenic Syndromes

Congenital myasthenic syndromes (CMS) are genetic disorders characterised by impaired neuromuscular transmission. This review provides an overview on CMS and highlights recent advances in the field, including novel CMS causative genes and improved therapeutic strategies. CMS due to mutations in SLC5A7 and SLC18A3, impairing the synthesis and recycling of acetylcholine, have recently been described. In addition, a novel group of CMS due to mutations in SNAP25B, SYT2, VAMP1, and UNC13A1 encoding molecules implicated in synaptic vesicles exocytosis has been characterised. The increasing number of presynaptic CMS exhibiting CNS manifestations along with neuromuscular weakness demonstrate that the myasthenia can be only a small part of a much more extensive disease phenotype. Moreover, the spectrum of glycosylation abnormalities has been increased with the report that GMPPB mutations can cause CMS, thus bridging myasthenic disorders with dystroglycanopathies. Finally, the discovery of COL13A1 mutations and laminin α5 deficiency has helped to draw attention to the role of extracellular matrix proteins for the formation and maintenance of muscle endplates. The benefit of β2-adrenergic agonists alone or combined with pyridostigmine or 3,4-Dyaminopiridine is increasingly being reported for different subtypes of CMS including AChR-deficiency and glycosylation abnormalities, thus expanding the therapeutic repertoire available.

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.

Wnt proteins contribute to neuromuscular junction formation through distinct signaling pathways

Understanding the developmental steps that shape formation of the neuromuscular junction (NMJ) connecting motoneurons to skeletal muscle fibers is crucial. Wnt morphogens are key players in the formation of this specialized peripheral synapse, but their individual and collaborative functions and downstream pathways remain poorly understood at the NMJ. Here, we demonstrate through Wnt4 and Wnt11 gain-of-function studies in cell culture or in mice that Wnts enhance acetylcholine receptor (AChR) clustering and motor axon outgrowth. By contrast, loss of Wnt11 or Wnt-dependent signaling in vivo decreases AChR clustering and motor nerve terminal branching. Both Wnt4 and Wnt11 stimulate AChR mRNA levels and AChR clustering downstream of activation of the β-catenin pathway. Strikingly, Wnt4 and Wnt11 co-immunoprecipitate with Vangl2, a core component of the planar cell polarity (PCP) pathway, which accumulates at embryonic NMJs. Moreover, mice bearing a Vangl2 loss-of-function mutation (loop-tail) exhibit fewer AChR clusters and overgrowth of motor axons bypassing AChR clusters. Together, our results provide genetic and biochemical evidence that Wnt4 and Wnt11 cooperatively contribute to mammalian NMJ formation through activation of both the canonical and Vangl2-dependent core PCP pathways.

Diverging roles for Lrp4 and Wnt signaling in neuromuscular synapse development during evolution

In this study, Remédio et al. use mice and zebrafish to show that muscle prepatterning in mammals and zebrafish is established by different mechanisms. Their findings demonstrate that Agrin/Lrp4/MuSK signaling plays an essential role in neuromuscular synapse formation in both fish and mammals, whereas Wnt signaling is dispensable.

Motor axons approach muscles that are prepatterned in the prospective synaptic region. In mice, prepatterning of acetylcholine receptors requires Lrp4, a LDLR family member, and MuSK, a receptor tyrosine kinase. Lrp4 can bind and stimulate MuSK, strongly suggesting that association between Lrp4 and MuSK, independent of additional ligands, initiates prepatterning in mice. In zebrafish, Wnts, which bind the Frizzled (Fz)-like domain in MuSK, are required for prepatterning, suggesting that Wnts may contribute to prepatterning and neuromuscular development in mammals. We show that prepatterning in mice requires Lrp4 but not the MuSK Fz-like domain. In contrast, prepatterning in zebrafish requires the MuSK Fz-like domain but not Lrp4. Despite these differences, neuromuscular synapse formation in zebrafish and mice share similar mechanisms, requiring Lrp4, MuSK, and neuronal Agrin but not the MuSK Fz-like domain or Wnt production from muscle. Our findings demonstrate that evolutionary divergent mechanisms establish muscle prepatterning in zebrafish and mice.

R-spondin 2 promotes acetylcholine receptor clustering at the neuromuscular junction via Lgr5

At the neuromuscular junction (NMJ), acetylcholine receptor (AChR) clustering is mediated by spinal motor neuron (SMN)-derived agrin and its receptors on the muscle, the low-density lipoprotein receptor-related protein 4 (LRP4) and muscle-specific receptor tyrosine kinase (MuSK). Additionally, AChR clustering is mediated by the components of the Wnt pathway. Laser capture microdissection of SMNs revealed that a secreted activator of Wnt signaling, R-spondin 2 (Rspo2), is highly expressed in SMNs. We found that Rspo2 is enriched at the NMJ, and that Rspo2 induces MuSK phosphorylation and AChR clustering. Rspo2 requires Wnt ligands, but not agrin, for promoting AChR clustering in cultured myotubes. Leucine-rich repeat-containing G-protein coupled receptor 5 (Lgr5), an Rspo2 receptor, is also accumulated at the NMJ, and is associated with MuSK via LRP4. Lgr5 is required for Rspo2-mediated AChR clustering in myotubes. In Rspo2-knockout mice, the number and density of AChRs at the NMJ are reduced. The Rspo2-knockout diaphragm has an altered ultrastructure with widened synaptic clefts and sparse synaptic vesicles. Frequency of miniature endplate currents is markedly reduced in Rspo2-knockout mice. To conclude, we demonstrate that Rspo2 and its receptor Lgr5 are Wnt-dependent and agrin-independent regulators of AChR clustering at the NMJ.

LRP4 in neuromuscular junction and bone development and diseases

Low-density lipoprotein receptor-related protein 4 (LRP4) is a member of the low-density lipoprotein receptor (LDLR) family. Recent studies have revealed multiple functions and complex signaling mechanisms of LRP4 in different organs and tissues. LPR4 mutation or malfunction has been implicated in neurological disorders including congenital myasthenic syndrome, myasthenia gravis, and diseases of bone or kidney. This article is part of a Special Issue entitled "Muscle Bone Interactions".

Combination of agrin and laminin increase acetylcholine receptor clustering and enhance functional neuromuscular junction formation In vitro

Clustering of acetylcholine receptors (AChR) at the postsynaptic membrane is a crucial step in the development of neuromuscular junctions (NMJ). During development and after denervation, aneural AChR clusters form on the sarcolemma. Recent studies suggest that these receptors are critical for guiding and initiating synaptogenesis. The aim of this study is to investigate the effect of agrin and laminin-1; agents with known AChR clustering activity; on NMJ formation and muscle maturation. Primary myoblasts were differentiated in vitro on collagen, laminin or collagen and laminin-coated surfaces in the presence or absence of agrin and laminin. The pretreated cells were then subject to innervation by PC12 cells. The number of neuromuscular junctions was assessed by immunocytochemical co-localization of AChR clusters and the presynaptic marker synaptophysin. Functional neuromuscular junctions were quantitated by analysis of the level of spontaneous as well as neuromuscular blocker responsive contractile activity and muscle maturation was assessed by the degree of myotube striation. Agrin alone did not prime muscle for innervation while a combination of agrin and laminin pretreatment increased the number of neuromuscular junctions formed and enhanced acetylcholine based neurotransmission and myotube striation. This study has direct clinical relevance for treatment of denervation injuries and creating functional neuromuscular constructs for muscle tissue repair.

Slit2 as a β-catenin/Ctnnb1-dependent retrograde signal for presynaptic differentiation

Neuromuscular junction formation requires proper interaction between motoneurons and muscle cells. β-Catenin (Ctnnb1) in muscle is critical for motoneuron differentiation; however, little is known about the relevant retrograde signal. In this paper, we dissected which functions of muscle Ctnnb1 are critical by an in vivo transgenic approach. We show that Ctnnb1 mutant without the transactivation domain was unable to rescue presynaptic deficits of Ctnnb1 mutation, indicating the involvement of transcription regulation. On the other hand, the cell-adhesion function of Ctnnb1 is dispensable. We screened for proteins that may serve as a Ctnnb1-directed retrograde factor and identified Slit2. Transgenic expression of Slit2 specifically in the muscle was able to diminish presynaptic deficits by Ctnnb1 mutation in mice. Slit2 immobilized on beads was able to induce synaptophysin puncta in axons of spinal cord explants. Together, these observations suggest that Slit2 serves as a factor utilized by muscle Ctnnb1 to direct presynaptic differentiation.

DOI:http://dx.doi.org/10.7554/eLife.07266.001

Motor nerves are like electrical wires that connect our spinal cord to the muscles in our body. These nerves communicate with muscles across a connection called the neuromuscular junction. To first form a neuromuscular junction, the motor nerves and muscles each produce molecular cues that tell each other to do their part to build a connection. Beta-catenin in the muscle is known to regulate motor nerve development. However, beta-catenin has two different roles: it helps to coordinate whether neighboring cells stick together, and it can regulate which genes are ‘transcribed’ to produce proteins. It was not known which of these roles is necessary for forming neuromuscular junctions.

Wu, Barik et al. now investigate this question by creating mice with mutant forms of beta-catenin in their muscles. Some mice had muscle beta-catenin that could not help cells stick together, and others had beta-catenin that could not control gene transcription. Only mutations that affected the ability of beta-catenin to control transcription caused abnormalities in the neuromuscular junction. However, these problems could be fixed by adding either normal beta-catenin or the mutant form that cannot help cells stick together.

Wu, Barik et al. then used molecular tools to explore which genes are turned on by beta-catenin. The experiments showed that beta-catenin causes muscle fibers to produce a protein called Slit2—a developmental cue that controls where neurons grow. Furthermore, the neuromuscular junction defects found in mice without beta-catenin in their muscles could be reduced by making the muscle fibers produce more Slit2. However, not all defects in beta-catenin mutant mice are rescued by Slit2. Future research is needed to identify other beta-catenin-controlled signals and to determine whether such a pathway is altered in neuromuscular disorders.

DOI:http://dx.doi.org/10.7554/eLife.07266.002

Canonical wnt signaling is required for commissural axon guidance

Morphogens have been identified as guidance cues for postcrossing commissural axons in the spinal cord. Shh has a dual effect on postcrossing commissural axons: a direct repellent effect mediated by Hhip as a receptor, and an indirect effect by shaping a Wnt activity gradient. Wnts were shown to be attractants for postcrossing commissural axons in both chicken and mouse embryos. In mouse, the effects of Wnts on axon guidance were concluded to depend on the planar cell polarity (PCP) pathway. Canonical Wnt signaling was excluded based on the absence of axon guidance defects in mice lacking Lrp6 which is an obligatory coreceptor for Fzd in canonical Wnt signaling. In the loss-of-function studies reported here, we confirmed a role for the PCP pathway in postcrossing commissural axon guidance also in the chicken embryo. However, taking advantage of the precise temporal control of gene silencing provided by in ovo RNAi, we demonstrate that canonical Wnt signaling is also required for proper guidance of postcrossing commissural axons in the developing spinal cord. Thus, axon guidance does not seem to depend on any one of the classical Wnt signaling pathways but rather involve a network of Wnt receptors and downstream components.

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.

MuSK frizzled-like domain is critical for mammalian neuromuscular junction formation and maintenance

The muscle-specific kinase MuSK is one of the key molecules orchestrating neuromuscular junction (NMJ) formation. MuSK interacts with the Wnt morphogens, through its Frizzled-like domain (cysteine-rich domain [CRD]). Dysfunction of MuSK CRD in patients has been recently associated with the onset of myasthenia, common neuromuscular disorders mainly characterized by fatigable muscle weakness. However, the physiological role of Wnt-MuSK interaction in NMJ formation and function remains to be elucidated. Here, we demonstrate that the CRD deletion of MuSK in mice caused profound defects of both muscle prepatterning, the first step of NMJ formation, and synapse differentiation associated with a drastic deficit in AChR clusters and excessive growth of motor axons that bypass AChR clusters. Moreover, adult MuSKΔCRD mice developed signs of congenital myasthenia, including severe NMJs dismantlement, muscle weakness, and fatigability. We also report, for the first time, the beneficial effects of lithium chloride, a reversible inhibitor of the glycogen synthase kinase-3, that rescued NMJ defects in MuSKΔCRD mice and therefore constitutes a novel therapeutic reagent for the treatment of neuromuscular disorders linked to Wnt-MuSK signaling pathway deficiency. Together, our data reveal that MuSK CRD is critical for NMJ formation and plays an unsuspected role in NMJ maintenance in adulthood.

Activation of the Wnt/β-catenin signaling cascade after traumatic nerve injury

Recent data have shown that preservation of the neuromuscular junction (NMJ) after traumatic nerve injury helps to improve functional recovery with surgical repair via matrix metalloproteinase-3 (MMP3) blockade. As such, we sought to explore additional pathways that may augment this response. Wnt3a has been shown to inhibit acetylcholine receptor (AChR) clustering via β-catenin-dependent signaling in the development of the NMJ. Therefore, we hypothesized that Wnt3a and β-catenin are associated with NMJ destabilization following traumatic denervation. A critical size nerve defect was created by excising a 10-mm segment of the sciatic nerve in mice. Denervated muscles were then harvested at multiple time points for immunofluorescence staining, quantitative real-time PCR, and western blot analysis for Wnt3a and β-catenin levels. Moreover, a novel Wnt/β-catenin transgenic reporter mouse line was utilized to support our hypothesis of Wnt activation after traumatic nerve injury. The expression of Wnt3a mRNA was significantly increased by 2 weeks post-injury and remained upregulated for 2 months. Additionally, β-catenin was activated at 2 months post-injury relative to controls. Correspondingly, immunohistochemical analysis of denervated transgenic mouse line TCF/Lef:H2B-GFP muscles demonstrated that the number of GFP-positive cells was increased at the motor endplate band. These collective data support that post-synaptic AChRs destabilize after denervation by a process that involves the Wnt/β-catenin pathway. As such, this pathway serves as a potential therapeutic target to prevent the motor endplate degeneration that occurs following traumatic nerve injury.

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.

Biomechanical strain vehicles for fibroblast-directed skeletal myoblast differentiation and myotube functionality in a novel coculture

Skeletal muscle functionality is governed by multiple stimuli, including cytokines and biomechanical strain. Fibroblasts embedded within muscle connective tissue respond to biomechanical strain by secreting cytokines that induce myoblast differentiation and, we hypothesize, regulate myotube function. A coculture was established to allow cross talk between fibroblasts in Bioflex wells and myoblasts on nondeformable coverslips situated above Bioflex wells. Cyclic short-duration strain (CSDS) modeling repetitive stress/injury, acyclic long-duration strain (ALDS) modeling manipulative therapy, and combined strain paradigms (CSDS + ALDS) were applied to fibroblasts. Nonstrained myoblasts in uniculture and coculture served as controls. After fibroblasts had induced myoblast differentiation, myotube contraction was assessed by perfusion of ACh (10(-11)-10(-3) M). CSDS-treated fibroblasts increased myotube contractile sensitivity vs. uniculture (P < 0.05). As contraction is dependent on ACh binding, expression and clustering of nicotinic ACh receptors (nAChRs) were measured. CSDS-treated fibroblasts increased nAChR expression (P < 0.05), which correlated with myotube contraction. ALDS-treated fibroblasts did not significantly affect contraction or nAChR expression. Agrin-treated myotubes were then used to design a computer algorithm to identify α-bungarotoxin-stained nAChR clusters. ALDS-treated fibroblasts increased nAChR clustering (P < 0.05), while CSDS-treated fibroblasts disrupted cluster formation. CSDS-treated fibroblasts produced nAChRs preferentially located in nonclustered regions (P < 0.05). Strain-activated fibroblasts mediate myotube differentiation with multiple functional phenotypes. Similar to muscle injury, CSDS-treated fibroblasts disrupted nAChR clusters and hypersensitized myotube contraction, while ALDS-treated fibroblasts aggregated nAChRs in large clusters, which may have important clinical implications. Cellular strategies aimed at improving muscle functionality, such as through biomechanical strain vehicles that activate fibroblasts to stabilize postsynaptic nAChRs on nearby skeletal muscle, may serve as novel targets in neuromuscular disorders.