Harmony Lost: Cell-Cell Communication at the Neuromuscular Junction in Motor Neuron Disease

Revisiting Glutamate Excitotoxicity in Amyotrophic Lateral Sclerosis and Age-Related Neurodegeneration

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron disorder. While there are five FDA-approved drugs for treating this disease, each has only modest benefits. To design new and more effective therapies for ALS, particularly for sporadic ALS of unknown and diverse etiologies, we must identify key, convergent mechanisms of disease pathogenesis. This review focuses on the origin and effects of glutamate-mediated excitotoxicity in ALS (the cortical hyperexcitability hypothesis), in which increased glutamatergic signaling causes motor neurons to become hyperexcitable and eventually die. We characterize both primary and secondary contributions to excitotoxicity, referring to processes taking place at the synapse and within the cell, respectively. 'Primary pathways' include upregulation of calcium-permeable AMPA receptors, dysfunction of the EAAT2 astrocytic glutamate transporter, increased release of glutamate from the presynaptic terminal, and reduced inhibition by cortical interneurons-all of which have been observed in ALS patients and model systems. 'Secondary pathways' include changes to mitochondrial morphology and function, increased production of reactive oxygen species, and endoplasmic reticulum (ER) stress. By identifying key targets in the excitotoxicity cascade, we emphasize the importance of this pathway in the pathogenesis of ALS and suggest that intervening in this pathway could be effective for developing therapies for this disease.

Unlocking the Complexity of Neuromuscular Diseases: Insights from Human Pluripotent Stem Cell-Derived Neuromuscular Junctions

Over the past 20 years, the use of pluripotent stem cells to mimic the complexities of the human neuromuscular junction has received much attention. Deciphering the key mechanisms underlying the establishment and maturation of this complex synapse has been driven by the dual goals of addressing developmental questions and gaining insight into neuromuscular disorders. This review aims to summarise the evolution and sophistication of in vitro neuromuscular junction models developed from the first differentiation of human embryonic stem cells into motor neurons to recent neuromuscular organoids. We also discuss the potential offered by these models to decipher different neuromuscular diseases characterised by defects in the presynaptic compartment, the neuromuscular junction, and the postsynaptic compartment. Finally, we discuss the emerging field that considers the use of these techniques in drug screening assay and the challenges they will face in the future.

Intercellular transmission of pathogenic proteins in ALS: Exploring the pathogenic wave

In patients with amyotrophic lateral sclerosis (ALS), disease symptoms and pathology typically spread in a predictable spatiotemporal pattern beginning at a focal site of onset and progressing along defined neuroanatomical tracts. Like other neurodegenerative diseases, ALS is characterized by the presence of protein aggregates in postmortem patient tissue. Cytoplasmic, ubiquitin-positive aggregates of TDP-43 are observed in approximately 97% of sporadic and familial ALS patients, while SOD1 inclusions are likely specific to cases of SOD1-ALS. Additionally, the most common subtype of familial ALS, caused by a hexanucleotide repeat expansion in the first intron of the C9orf72 gene (C9-ALS), is further characterized by the presence of aggregated dipeptide repeat proteins (DPRs). As we will describe, cell-to-cell propagation of these pathological proteins tightly correlates with the contiguous spread of disease. While TDP-43 and SOD1 are capable of seeding protein misfolding and aggregation in a prion-like manner, C9orf72 DPRs appear to induce (and transmit) a 'disease state' more generally. Multiple mechanisms of intercellular transport have been described for all of these proteins, including anterograde and retrograde axonal transport, extracellular vesicle secretion, and macropinocytosis. In addition to neuron-to-neuron transmission, transmission of pathological proteins occurs between neurons and glia. Given that the spread of ALS disease pathology corresponds with the spread of symptoms in patients, the various mechanisms by which ALS-associated protein aggregates propagate through the central nervous system should be closely examined.

Long-Term SMN- and Ncald-ASO Combinatorial Therapy in SMA Mice and NCALD-ASO Treatment in hiPSC-Derived Motor Neurons Show Protective Effects

For SMA patients with only two SMN2 copies, available therapies might be insufficient to counteract lifelong motor neuron (MN) dysfunction. Therefore, additional SMN-independent compounds, supporting SMN-dependent therapies, might be beneficial. Neurocalcin delta (NCALD) reduction, an SMA protective genetic modifier, ameliorates SMA across species. In a low-dose SMN-ASO-treated severe SMA mouse model, presymptomatic intracerebroventricular (i.c.v.) injection of Ncald-ASO at postnatal day 2 (PND2) significantly ameliorates histological and electrophysiological SMA hallmarks at PND21. However, contrary to SMN-ASOs, Ncald-ASOs show a shorter duration of action limiting a long-term benefit. Here, we investigated the longer-term effect of Ncald-ASOs by additional i.c.v. bolus injection at PND28. Two weeks after injection of 500 µg Ncald-ASO in wild-type mice, NCALD was significantly reduced in the brain and spinal cord and well tolerated. Next, we performed a double-blinded preclinical study combining low-dose SMN-ASO (PND1) with 2× i.c.v. Ncald-ASO or CTRL-ASO (100 µg at PND2, 500 µg at PND28). Ncald-ASO re-injection significantly ameliorated electrophysiological defects and NMJ denervation at 2 months. Moreover, we developed and identified a non-toxic and highly efficient human NCALD-ASO that significantly reduced NCALD in hiPSC-derived MNs. This improved both neuronal activity and growth cone maturation of SMA MNs, emphasizing the additional protective effect of NCALD-ASO treatment.

Post-synaptic specialization of the neuromuscular junction: junctional folds formation, function, and disorders

Post-synaptic specialization is critical to the neurotransmitter release and action potential conduction. The neuromuscular junctions (NMJs) are the synapses between the motor neurons and muscle cells and have a more specialized post-synaptic membrane than synapses in the central nervous system (CNS). The sarcolemma within NMJ folded to form some invagination portions called junctional folds (JFs), and they have important roles in maintaining the post-synaptic membrane structure. The NMJ formation and the acetylcholine receptor (AChR) clustering signal pathway have been extensively studied and reviewed. Although it has been suggested that JFs are related to maintaining the safety factor of neurotransmitter release, the formation mechanism and function of JFs are still unclear. This review will focus on the JFs about evolution, formation, function, and disorders. Anticipate understanding of where they are coming from and where we will study in the future.

A link between agrin signalling and Cav3.2 at the neuromuscular junction in spinal muscular atrophy

SMN protein deficiency causes motoneuron disease spinal muscular atrophy (SMA). SMN-based therapies improve patient motor symptoms to variable degrees. An early hallmark of SMA is the perturbation of the neuromuscular junction (NMJ), a synapse between a motoneuron and muscle cell. NMJ formation depends on acetylcholine receptor (AChR) clustering triggered by agrin and its co-receptors lipoprotein receptor-related protein 4 (LRP4) and transmembrane muscle-specific kinase (MuSK) signalling pathway. We have previously shown that flunarizine improves NMJs in SMA model mice, but the mechanisms remain elusive. We show here that flunarizine promotes AChR clustering in cell-autonomous, dose- and agrin-dependent manners in C2C12 myotubes. This is associated with an increase in protein levels of LRP4, integrin-beta-1 and alpha-dystroglycan, three agrin co-receptors. Furthermore, flunarizine enhances MuSK interaction with integrin-beta-1 and phosphotyrosines. Moreover, the drug acts on the expression and splicing of Agrn and Cacna1h genes in a muscle-specific manner. We reveal that the Cacna1h encoded protein Cav3.2 closely associates in vitro with the agrin co-receptor LRP4. In vivo, it is enriched nearby NMJs during neonatal development and the drug increases this immunolabelling in SMA muscles. Thus, flunarizine modulates key players of the NMJ and identifies Ca_v3.2 as a new protein involved in the NMJ biology.

SMN controls neuromuscular junction integrity through U7 snRNP

The neuromuscular junction (NMJ) is an essential synapse whose loss is a key hallmark of the neurodegenerative disease spinal muscular atrophy (SMA). Here, we show that activity of the SMA-determining SMN protein in the assembly of U7 small nuclear ribonucleoprotein (snRNP)—which functions in the 3′-end processing of replication-dependent histone mRNAs—is required for NMJ integrity. Co-expression of U7-specific Lsm10 and Lsm11 proteins selectively enhances U7 snRNP assembly, corrects histone mRNA processing defects, and rescues key structural and functional abnormalities of neuromuscular pathology in SMA mice—including NMJ denervation, decreased synaptic transmission, and skeletal muscle atrophy. Furthermore, U7 snRNP dysfunction drives selective loss of the synaptic organizing protein Agrin at NMJs innervating vulnerable muscles of SMA mice. These findings reveal a direct contribution of U7 snRNP dysfunction to neuromuscular pathology in SMA and suggest a role for histone gene regulation in maintaining functional synaptic connections between motor neurons and muscles.

NMJ denervation is a hallmark of SMA. Through selective restoration of U7 snRNP biogenesis in SMA mice, Tisdale et al. reveal a role for SMN-mediated U7 snRNP assembly and histone mRNA processing in controlling NMJ integrity through Agrin expression, uncovering RNA-mediated disease mechanisms and linking U7 function to neuromuscular development.

Recognising the potential of large animals for modelling neuromuscular junction physiology and disease

The aetiology and pathophysiology of many diseases of the motor unit remain poorly understood and the role of the neuromuscular junction (NMJ) in this group of disorders is particularly overlooked, especially in humans, when these diseases are comparatively rare. However, elucidating the development, function and degeneration of the NMJ is essential to uncover its contribution to neuromuscular disorders, and to explore potential therapeutic avenues to treat these devastating diseases. Until now, an understanding of the role of the NMJ in disease pathogenesis has been hindered by inherent differences between rodent and human NMJs: stark contrasts in body size and corresponding differences in associated axon length underpin some of the translational issues in animal models of neuromuscular disease. Comparative studies in large mammalian models, including examination of naturally occurring, highly prevalent animal diseases and evaluation of their treatment, might provide more relevant insights into the pathogenesis and therapy of equivalent human diseases. This review argues that large animal models offer great potential to enhance our understanding of the neuromuscular system in health and disease, and in particular, when dealing with diseases for which nerve length dependency might underly the pathogenesis.

Three-dimensional in vitro models of neuromuscular tissue

Skeletal muscle is a dynamic tissue in which homeostasis and function are guaranteed by a very defined three-dimensional organization of myofibers in respect to other non-muscular components, including the extracellular matrix and the nervous network. In particular, communication between myofibers and the nervous system is essential for the overall correct development and function of the skeletal muscle. A wide range of chronic, acute and genetic-based human pathologies that lead to the alteration of muscle function are associated with modified preservation of the fine interaction between motor neurons and myofibers at the neuromuscular junction. Recent advancements in the development of in vitro models for human skeletal muscle have shown that three-dimensionality and integration of multiple cell types are both key parameters required to unveil pathophysiological relevant phenotypes. Here, we describe recent achievement reached in skeletal muscle modeling which used biomaterials for the generation of three-dimensional constructs of myotubes integrated with motor neurons.

Current Progress in the Creation, Characterization, and Application of Human Stem Cell-derived in Vitro Neuromuscular Junction Models

Human pluripotent stem cells (PSCs) such as embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) are of great value for studying developmental processes, disease modeling, and drug testing. One area in which the use of human PSCs has become of great interest in recent years is for in vitro models of the neuromuscular junction (NMJ). The NMJ is a synapse at which a motor neuron releases acetylcholine to bind to skeletal muscle and stimulate contraction. Degeneration of the NMJ and subsequent loss of muscle function is a common feature of many neuromuscular diseases such as myasthenia gravis, spinal muscular atrophy, and amyotrophic lateral sclerosis. In order to develop new therapies for patients with neuromuscular diseases, it is essential to understand mechanisms taking place at the NMJ. However, we have limited ability to study the NMJ in living human patients, and animal models are limited by physiological relevance. Therefore, an in vitro model of the NMJ consisting of human cells is of great value. The use of stem cells for in vitro NMJ models is still in progress and requires further optimization in order to yield reliable, reproducible results. The objective of this review is (1) to outline the current progress towards fully PSC-derived in vitro co-culture models of the human NMJ and (2) to discuss future directions and challenges that must be overcome in order to create reproducible fully PSC-derived models that can be used for developmental studies, disease modeling, and drug testing.

Microfluidics-Based Single-Cell Research for Intercellular Interaction

Intercellular interaction between cell-cell and cell-ECM is critical to numerous biology and medical studies, such as stem cell differentiation, immunotherapy and tissue engineering. Traditional methods employed for delving into intercellular interaction are limited by expensive equipment and sophisticated procedures. Microfluidics technique is considered as one of the powerful measures capable of precisely capturing and manipulating cells and achieving low reagent consumption and high throughput with decidedly integrated functional components. Over the past few years, microfluidics-based systems for intercellular interaction study at a single-cell level have become frequently adopted. This review focuses on microfluidic single-cell studies for intercellular interaction in a 2D or 3D environment with a variety of cell manipulating techniques and applications. The challenges to be overcome are highlighted.

The Neuromuscular Junction: Roles in Aging and Neuromuscular Disease

The neuromuscular junction (NMJ) is a specialized synapse that bridges the motor neuron and the skeletal muscle fiber and is crucial for conversion of electrical impulses originating in the motor neuron to action potentials in the muscle fiber. The consideration of contributing factors to skeletal muscle injury, muscular dystrophy and sarcopenia cannot be restricted only to processes intrinsic to the muscle, as data show that these conditions incur denervation-like findings, such as fragmented NMJ morphology and corresponding functional changes in neuromuscular transmission. Primary defects in the NMJ also influence functional loss in motor neuron disease, congenital myasthenic syndromes and myasthenia gravis, resulting in skeletal muscle weakness and heightened fatigue. Such findings underscore the role that the NMJ plays in neuromuscular performance. Regardless of cause or effect, functional denervation is now an accepted consequence of sarcopenia and muscle disease. In this short review, we provide an overview of the pathologic etiology, symptoms, and therapeutic strategies related to the NMJ. In particular, we examine the role of the NMJ as a disease modifier and a potential therapeutic target in neuromuscular injury and disease.

Clenbuterol-sensitive delayed outward potassium currents in a cell model of spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disease caused by polyglutamine (polyQ) expansions in the androgen receptor (AR) gene. SBMA is characterized by selective dysfunction and degeneration of motor neurons in the brainstem and spinal cord through still unclear mechanisms in which ion channel modulation might play a central role as for other neurodegenerative diseases. The beta2-adrenergic agonist clenbuterol was observed to ameliorate the SBMA phenotype in mice and patient-derived myotubes. However, the underlying molecular mechanism has yet to be clarified. Here, we unveil that ionic current alterations induced by the expression of polyQ-expanded AR in motor neuron-derived MN-1 cells are attenuated by the administration of clenbuterol. Our combined electrophysiological and pharmacological approach allowed us to reveal that clenbuterol modifies delayed outward potassium currents. Overall, we demonstrated that the protection provided by clenbuterol restores the normal function through the modulation of K_V2-type outward potassium currents, possibly contributing to the protective effect on motor neuron toxicity in SBMA.

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.