Skeletal muscle in motor neuron diseases: therapeutic target and delivery route for potential treatments

Endurance exercise has a negative impact on the onset of SOD1-G93A ALS in female mice and affects the entire skeletal muscle-motor neuron axis

Background

Amyotrophic lateral sclerosis (ALS) is a fatal neuromuscular disease characterized by the degeneration of motor neurons that leads to muscle wasting and atrophy. Epidemiological and experimental evidence suggests a causal relationship between ALS and physical activity (PA). However, the impact of PA on motor neuron loss and sarcopenia is still debated, probably because of the heterogeneity and intensities of the proposed exercises. With this study, we aimed to clarify the effect of intense endurance exercise on the onset and progression of ALS in the SOD1-G93A mouse model.

Methods

We randomly selected four groups of twelve 35-day-old female mice. SOD1-G93A and WT mice underwent intense endurance training on a motorized treadmill for 8 weeks, 5 days a week. During the training, we measured muscle strength, weight, and motor skills and compared them with the corresponding sedentary groups to define the disease onset. At the end of the eighth week, we analyzed the skeletal muscle-motor neuron axis by histological and molecular techniques.

Results

Intense endurance exercise anticipates the onset of the disease by 1 week (age of the onset: trained SOD1-G93A = 63.17 ± 2.25 days old; sedentary SOD1-G93A = 70.75 ± 2.45 days old). In SOD1-G93A mice, intense endurance exercise hastens the muscular switch to a more oxidative phenotype and worsens the denervation process by dismantling neuromuscular junctions in the tibialis anterior, enhancing the Wallerian degeneration in the sciatic nerve, and promoting motor neuron loss in the spinal cord. The training exacerbates neuroinflammation, causing immune cell infiltration in the sciatic nerve and a faster activation of astrocytes and microglia in the spinal cord.

Conclusion

Intense endurance exercise, acting on skeletal muscles, worsens the pathological hallmarks of ALS, such as denervation and neuroinflammation, brings the onset forward, and accelerates the progression of the disease. Our findings show the potentiality of skeletal muscle as a target for both prognostic and therapeutic strategies; the preservation of skeletal muscle health by specific intervention could counteract the dying-back process and protect motor neurons from death. The physiological characteristics and accessibility of skeletal muscle further enhance its appeal as a therapeutic target.

Scalable, optically-responsive human neuromuscular junction model reveals convergent mechanisms of synaptic dysfunction in familial ALS

Neuromuscular junctions (NMJs) are specialized synapses that mediate communication between motor neurons and skeletal muscles and are essential for movement. The degeneration of this system can lead to symptoms observed in neuromuscular and motor neuron diseases. Studying these synapses and their degeneration has proven challenging. Prior NMJ studies heavily relied upon the use of mouse, chick, or isolated primary human cells, which have demonstrated limited fidelity for disease modeling. To enable the study of NMJ dysfunction and model genetic diseases, we, and others, have developed methods to generate human NMJs from pluripotent stem cells (PSCs), embryonic stem cells, and induced pluripotent stem cells. However, published studies have highlighted technical limitations associated with these complex in vitro NMJ models. In this study, we developed a robust PSC-derived motor neuron and skeletal muscle co-culture method, and demonstrated its sensitivity in modeling motor neuron disease. Our method spontaneously and reproducibly forms human NMJs. We developed multiwell-multielectrode array (MEA) parameters to quantify the activity of PSC-derived skeletal muscles, as well as measured the electrophysiological activity of functional human PSC-derived NMJs. We further leveraged our method to morphologically and functionally assess NMJs from the familial amyotrophic lateral sclerosis (fALS) PSCs, C9orf72 hexanucleotide (G4C2)n repeat expansion (HRE), SOD1 A5V , and TDP43 G298S to define the reproducibility and sensitivity of our system. We observed a significant decrease in the numbers and activity of PSC-derived NMJs developed from the different ALS lines compared to their respective controls. Furthermore, we evaluated a therapeutic candidate undergoing clinical trials and observed a variant-dependent rescue of functionality of NMJs. Our newly developed method provides a platform for the systematic investigation of genetic causes of NMJ neurodegeneration and highlights the need for therapeutic avenues to consider patient genotype.

The Cellular and Molecular Signature of ALS in Muscle

Amyotrophic lateral sclerosis is a disease affecting upper and lower motor neurons. Although motor neuron death is the core event of ALS pathology, it is increasingly recognized that other tissues and cell types are affected in the disease, making potentially major contributions to the occurrence and progression of pathology. We review here the known cellular and molecular characteristics of muscle tissue affected by ALS. Evidence of toxicity in skeletal muscle tissue is considered, including metabolic dysfunctions, impaired proteostasis, and deficits in muscle regeneration and RNA metabolism. The role of muscle as a secretory organ, and effects on the skeletal muscle secretome are also covered, including the increase in secretion of toxic factors or decrease in essential factors that have consequences for neuronal function and survival.

FUS Mutation Causes Disordered Lipid Metabolism in Skeletal Muscle Associated with ALS

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease characterized by dysfunction of the upper and lower motor neurons resulting in muscle weakness and wasting. Recently, several studies on ALS patients and ALS animal models indicated that intramuscular toxicity played a role in ALS disease progression; however, the mechanisms driving this are unknown. In this study, we explored the possible dysfunction of lipid metabolism in myocytes associated with ALS. Initially, skeletal muscle from 41 ALS patients, as well as 53 non-ALS control subjects, was investigated, and we identified that lipid droplet accumulation in the muscle fibers of ALS patients was significantly increased, especially in patients with FUS mutations. A myoblast (C2C12) cell line expressing mutant FUS (FUS-K510Q) was able to induce lipid droplet accumulation and mitochondrial dysfunction. Consistently, transgenic flies expressing FUS-K510Q under a muscle-specific driver showed elevated triglyceride levels in the flight muscles, as well as locomotor defects. Biochemical analysis of C2C12 cells and fly muscle tissues showed upregulation of PLIN2, and downregulation of ATGL and CPT1A, indicating inhibition of lipolysis and fatty acid β-oxidation in muscle cells with FUS mutations. Our study provided a potential explanation for the pathogenesis associated with lipid droplets accumulating in skeletal muscle in ALS. Our data also suggested that disordered lipid metabolism and mitochondrial dysfunction play a crucial role in intramuscular toxicity in ALS.

Neuroprotective effects of violacein in a model of inherited amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive death of motor neurons and muscle atrophy, with defective neuron-glia interplay and emergence of aberrant glial phenotypes having a role in disease pathology. Here, we have studied if the pigment violacein with several reported protective/antiproliferative properties may control highly neurotoxic astrocytes (AbAs) obtained from spinal cord cultures of symptomatic hSOD1G93A rats, and if it could be neuroprotective in this ALS experimental model. At concentrations lower than those reported as protective, violacein selectively killed aberrant astrocytes. Treatment of hSOD1G93A rats with doses equivalent to the concentrations that killed AbAs caused a marginally significant delay in survival, partially preserved the body weight and soleus muscle mass and improved the integrity of the neuromuscular junction. Reduced motor neuron death and glial reactivity was also found and likely related to decreased inflammation and matrix metalloproteinase-2 and -9. Thus, in spite that new experimental designs aimed at extending the lifespan of hSOD1G93A rats are needed, improvements observed upon violacein treatment suggest a significant therapeutic potential that deserves further studies.

Glucose clearance and uptake is increased in the SOD1G93A mouse model of amyotrophic lateral sclerosis through an insulin-independent mechanism

Metabolic disturbances are associated with the progression of the neurodegenerative disorder, amyotrophic lateral sclerosis (ALS). However, the molecular events that drive energy imbalances in ALS are not completely understood. In this study, we aimed to elucidate deficits in energy homeostasis in the SOD1^G93A mouse model of ALS. SOD1^G93A mice and their wild-type littermates underwent indirect calorimetry and intraperitoneal glucose/insulin tolerance tests at both the onset and mid-symptomatic stages of the disease. Glucose uptake and the plasma glucoregulatory hormone profiles were analyzed. Pancreatic islet cell mass and function were assessed by measuring hormone concentrations and secretion in isolated islets, and pancreatic α- and β-cell immunoreactive areas. Finally, we profiled liver glycogen metabolism by measuring glucagon concentrations and liver metabolic gene expressions. We identified that mid-symptomatic SOD1^G93A mice have increased oxygen consumption and faster exogenous glucose uptake, despite presenting with normal insulin tolerance. The capacity for pancreatic islets to secrete insulin appears intact, however, islet cell insulin concentrations and β-cell mass were reduced. Fasting glucose homeostasis was also disturbed, along with increased liver glycogen stores, despite elevated circulating glucagon, suggesting that glucagon signaling is impaired. Metabolic gene expression profiling of livers indicated that glucose cannot be utilized efficiently in SOD1^G93A mice. Overall, we demonstrate that glucose homeostasis and uptake are altered in SOD1^G93A mice, which is linked to an increase in insulin-independent glucose uptake, and a loss of β-cells, insulin production, and glucagon sensitivity. This suggests that the hormonal regulation of glucose concentrations may contribute to the progression of disease in this ALS mouse model.

MyomiRs and their multifaceted regulatory roles in muscle homeostasis and amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by loss of both upper and lower motor neurons (MNs). The main clinical features of ALS are motor function impairment, progressive muscle weakness, muscle atrophy and, ultimately, paralysis. Intrinsic skeletal muscle deterioration plays a crucial role in the disease and contributes to ALS progression. Currently, there are no effective treatments for ALS, highlighting the need to obtain a deeper understanding of the molecular events underlying degeneration of both MNs and muscle tissue, with the aim of developing successful therapies. Muscle tissue is enriched in a group of microRNAs called myomiRs, which are effective regulators of muscle homeostasis, plasticity and myogenesis in both physiological and pathological conditions. After providing an overview of ALS pathophysiology, with a focus on the role of skeletal muscle, we review the current literature on myomiR network dysregulation as a contributing factor to myogenic perturbations and muscle atrophy in ALS. We argue that, in view of their critical regulatory function at the interface between MNs and skeletal muscle fiber, myomiRs are worthy of further investigation as potential molecular targets of therapeutic strategies to improve ALS symptoms and counteract disease progression.

Dysregulation of Muscle-Specific MicroRNAs as Common Pathogenic Feature Associated with Muscle Atrophy in ALS, SMA and SBMA: Evidence from Animal Models and Human Patients

Motor neuron diseases (MNDs) are neurodegenerative disorders characterized by upper and/or lower MN loss. MNDs include amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), and spinal and bulbar muscular atrophy (SBMA). Despite variability in onset, progression, and genetics, they share a common skeletal muscle involvement, suggesting that it could be a primary site for MND pathogenesis. Due to the key role of muscle-specific microRNAs (myomiRs) in skeletal muscle development, by real-time PCR we investigated the expression of miR-206, miR-133a, miR-133b, and miR-1, and their target genes, in G93A-SOD1 ALS, Δ7SMA, and KI-SBMA mouse muscle during disease progression. Further, we analyzed their expression in serum of SOD1-mutated ALS, SMA, and SBMA patients, to demonstrate myomiR role as noninvasive biomarkers. Our data showed a dysregulation of myomiRs and their targets, in ALS, SMA, and SBMA mice, revealing a common pathogenic feature associated with muscle impairment. A similar myomiR signature was observed in patients’ sera. In particular, an up-regulation of miR-206 was identified in both mouse muscle and serum of human patients. Our overall findings highlight the role of myomiRs as promising biomarkers in ALS, SMA, and SBMA. Further investigations are needed to explore the potential of myomiRs as therapeutic targets for MND treatment.

Ultrastructural characterization of peripheral denervation in a mouse model of Type III spinal muscular atrophy

Spinal muscular atrophy (SMA) is a heritable, autosomal recessive neuromuscular disorder characterized by a loss of the survival of motor neurons (SMN) protein, which leads to degeneration of lower motor neurons, and muscle atrophy. Despite SMA being nosographically classified as a motor neuron disease, recent advances indicate that peripheral alterations at the level of the neuromuscular junction (NMJ), involving the muscle, and axons of the sensory-motor system, occur early, and may even precede motor neuron loss. In the present study, we used a mouse model of slow progressive (type III) SMA, whereby the absence of the mouse SMN protein is compensated by the expression of two human genes (heterozygous SMN1A2G, and SMN2). This leads to late disease onset and prolonged survival, which allows for dissecting slow degenerative steps operating early in SMA pathogenesis. In this purely morphological study carried out at transmission electron microscopy, we extend the examination of motor neurons and proximal axons towards peripheral components, including distal axons, muscle fibers, and also muscle spindles. We document remarkable ultrastructural alterations being consistent with early peripheral denervation in SMA, which may shift the ultimate anatomical target in neuromuscular disease from the spinal cord towards the muscle. This concerns mostly mitochondrial alterations within distal axons and muscle, which are quantified here through ultrastructural morphometry. The present study is expected to provide a deeper knowledge of early pathogenic mechanisms in SMA.

Neurodegeneration and axonal mRNA transportation

The prevalence of neurodegenerative diseases is accelerating in rapidly aging global population. Novel and effective diagnostic and therapeutic methods are required to tackle the global issue of neurodegeneration in the future. A better understanding of the potential molecular mechanism causing neurodegeneration can shed light on dysfunctional processes in diseased neurons, which can pave the way to design and synthesize novel targets for early diagnosis during the asymptomatic phase of the disease. Abnormal protein aggregation is a hallmark of neurodegenerative diseases which can hamper transportation of cargoes into axons. Recent evidence suggests that disruption of local protein synthesis has been observed in neurodegenerative diseases. Because of their highly asymmetric structure, highly polarized neurons require trafficking of cargoes from the cell body to different subcellular regions to meet the extensive demands of cellular physiology. Localization of mRNAs and subsequent local translation to corresponding proteins in axons is a mechanism which allows neurons to rapidly respond to external stimuli as well as establishing neuronal networks by synthesizing proteins on demand. Axonal protein synthesis is required for axon guidance, synapse formation and plasticity, axon maintenance and regeneration in response to injury. Different types of excitatory and inhibitory neurons in the central and peripheral nervous systems have been shown to localize mRNA. Rising evidence suggests that the repertoire of localizing mRNA in axons can change during aging, indicating a connection between axonal mRNA trafficking and aging diseases such as neurodegeneration. Here, I briefly review the latest findings on the importance of mRNA localization and local translation in neurons and the consequences of their disruption in neurodegenerative diseases. In addition, I discuss recent evidence that dysregulation of mRNA localization and local protein translation can contribute to the formation of neurodegenerative diseases such as Alzheimer's disease, Amyotrophic Lateral Sclerosis, and Spinal Muscular Atrophy. In addition, I discuss recent findings on mRNAs localizing to mitochondria in neurodegeneration.

Cell-Clearing Systems Bridging Repeat Expansion Proteotoxicity and Neuromuscular Junction Alterations in ALS and SBMA

The coordinated activities of autophagy and the ubiquitin proteasome system (UPS) are key to preventing the aggregation and toxicity of misfold-prone proteins which manifest in a number of neurodegenerative disorders. These include proteins which are encoded by genes containing nucleotide repeat expansions. In the present review we focus on the overlapping role of autophagy and the UPS in repeat expansion proteotoxicity associated with chromosome 9 open reading frame 72 (C9ORF72) and androgen receptor (AR) genes, which are implicated in two motor neuron disorders, amyotrophic lateral sclerosis (ALS) and spinal-bulbar muscular atrophy (SBMA), respectively. At baseline, both C9ORF72 and AR regulate autophagy, while their aberrantly-expanded isoforms may lead to a failure in both autophagy and the UPS, further promoting protein aggregation and toxicity within motor neurons and skeletal muscles. Besides proteotoxicity, autophagy and UPS alterations are also implicated in neuromuscular junction (NMJ) alterations, which occur early in both ALS and SBMA. In fact, autophagy and the UPS intermingle with endocytic/secretory pathways to regulate axonal homeostasis and neurotransmission by interacting with key proteins which operate at the NMJ, such as agrin, acetylcholine receptors (AChRs), and adrenergic beta2 receptors (B2-ARs). Thus, alterations of autophagy and the UPS configure as a common hallmark in both ALS and SBMA disease progression. The findings here discussed may contribute to disclosing overlapping molecular mechanisms which are associated with a failure in cell-clearing systems in ALS and SBMA.

Sonographic and 3T-MRI-based evaluation of the tongue in ALS

A few systematic imaging studies employing ultrasound (HRUS) and magnetic resonance imaging (MRI) have suggested tongue measures to aid in diagnosis of amyotrophic lateral sclerosis (ALS). The relationship between structural tongue alterations and the ALS patients' bulbar and overall motor function has not yet been elucidated. We here thus aimed to understand how in-vivo tongue alterations relate to motor function and motor function evolution over time in ALS. Our study included 206 ALS patients and 104 age- and sex-matched controls that underwent HRUS and 3T MRI of the tongue at baseline. Sonographic measures comprised coronal tongue echointensity, area, height, width and height/width ratio, while MRI measures comprised sagittal T1 intensity, tongue area, position and shape. Imaging-derived markers were related to baseline and longitudinal bulbar and overall motor function. Baseline T1 intensity was lower in ALS patients with more severe bulbar involvement at baseline. Smaller baseline coronal (HRUS) and sagittal (MRI) tongue area, smaller coronal height (HRUS) and width (HRUS) as well as more rounded sagittal tongue shape predicated more rapid functional impairment - not only of bulbar, but also of overall motor function - in ALS. Our results suggest that in-vivo sonography und MRI tongue measures could aid as biomarkers to reflect bulbar and motor function impairment.

Role of Dynein and Dynactin (DCTN-1) in Neurodegenerative Diseases

The pathophysiology and concept of degeneration in central nervous system is very complex and overwhelming at times. There is a complex mechanism which exists among different molecules in the cytoplasm of cell bodies of neurons, antegrade and retrograde axonal transport of cargoes and accumulation of certain substances and proteins which can influence the excitatory neurotransmitter like glutamate initiating the process of neurodegeneration. Neurons have extensive processes and communication between those processes and the cell body is crucial to neuronal function, viability and survival over time with progression of age. Researchers believe neurons are uniquely dependent on microtubule-based cargo transport. There is enough evidence to support that deficits in retrograde axonal transport contribute to pathogenesis in multiple neurodegenerative diseases. Cytoplasmic dynein and its regulation by Dynactin (DCTN1) is the major molecular motor cargo involved in autophagy, mitosis and neuronal cell survival. Mutation in dynactin gene located in 2p13.1,is indeed studied very extensively and is considered to be involved directly or indirectly to various conditions like Perry syndrome, familial and sporadic Amyotrophic lateral sclerosis, Hereditary spastic paraplegia, Spinocerebellar Ataxia (SCA-5), Huntingtons disease, Alzheimers disease, Charcot marie tooth disease, Hereditary motor neuropathy 7B, prion disease, parkinsons disease, malformation of cortical development, polymicrogyria to name a few with exception of Multiple Sclerosis (MS).

Role of Mutant TBP in Regulation of Myogenesis on Muscle Satellite Cells

In polyglutamine (PolyQ) diseases, mutant proteins cause not only neurological problems but also peripheral tissue abnormalities. Among all systemic damages, skeletal muscle dystrophy is the severest. Previously by studying knock-in (KI) mouse models of spinal cerebellar ataxia 17 (SCA17), it was found that mutant TATA box binding protein (TBP) decreases its interaction with myogenic differentiation antigen, thus reducing the expression of skeletal muscle structural proteins and resulting in muscle degeneration. In this paper, the role of mutant TBP in myogenesis was investigated. Single myofibers were isolated from tibialis anterior muscles of wild type (WT) and SCA17KI mice. The 1TBP18 staining confirmed the expression of mutant TBP in muscle satellite cells in SCA17KI mice. In the BaCl₂-induced TA muscle injury, H&E cross-section staining showed no significant change in myofibril size before and after BaCl₂ treatment, and there was no significant difference in centralized nuclei between WT and SCA17KI mice, suggesting that mutant TBP had no significant effect on muscle regeneration. In the cultured primary myoblasts from WT and SCA17KI mice in vitro, representative BrdU immunostaining showed no significant difference in proliferation of muscle satellite cells. The primary myoblasts were then induced to differentiate and immunostained for eMyHC, and the staining showed there was no significant difference in differentiation of primary myoblasts between WT and SCA1KI mice. Our findings confirmed that mutant TBP had no significant effect on myogenesis.

ALS-Associated Endoplasmic Reticulum Proteins in Denervated Skeletal Muscle: Implications for Motor Neuron Disease Pathology

Alpha-motoneurons and muscle fibres are structurally and functionally interdependent. Both cell types particularly rely on endoplasmic reticulum (ER/SR) functions. Mutations of the ER proteins VAPB, SigR1 and HSP27 lead to hereditary motor neuron diseases (MNDs). Here, we determined the expression profile and localization of these ER proteins/chaperons by immunohistochemistry and immunoblotting in biopsy and autopsy muscle tissue of patients with amyotrophic lateral sclerosis (ALS) and other neurogenic muscular atrophies (NMAs) and compared these patterns to mouse models of neurogenic muscular atrophy. Postsynaptic neuromuscular junction staining for VAPB was intense in normal human and mouse muscle and decreased in denervated Nmd^2J mouse muscle fibres. In contrast, VAPB levels together with other chaperones and autophagy markers were increased in extrasynaptic regions of denervated muscle fibres of patients with MNDs and other NMAs, especially at sites of focal myofibrillar disintegration (targets). These findings did not differ between NMAs due to ALS and other causes. G93A-SOD1 mouse muscle fibres showed a similar pattern of protein level increases in denervated muscle fibres. In addition, they showed globular VAPB-immunoreactive structures together with misfolded SOD1 protein accumulations, suggesting a primary myopathic change. Our findings indicate that altered expression and localization of these ER proteins and autophagy markers are part of the dynamic response of muscle fibres to denervation. The ER is particularly prominent and vulnerable in both muscle fibres and alpha-motoneurons. Thus, ER pathology could contribute to the selective build-up of degenerative changes in the neuromuscular axis in MNDs.

Deficiency of the Survival of Motor Neuron Protein Impairs mRNA Localization and Local Translation in the Growth Cone of Motor Neurons

Spinal muscular atrophy (SMA) is a neurodegenerative disease primarily affecting spinal motor neurons. It is caused by reduced levels of the survival of motor neuron (SMN) protein, which plays an essential role in the biogenesis of spliceosomal small nuclear ribonucleoproteins in all tissues. The etiology of the specific defects in the motor circuitry in SMA is still unclear, but SMN has also been implicated in mediating the axonal localization of mRNA-protein complexes, which may contribute to the axonal degeneration observed in SMA. Here, we report that SMN deficiency severely disrupts local protein synthesis within neuronal growth cones. We also identify the cytoskeleton-associated growth-associated protein 43 (GAP43) mRNA as a new target of SMN and show that motor neurons from SMA mouse models have reduced levels ofGAP43mRNA and protein in axons and growth cones. Importantly, overexpression of two mRNA-binding proteins, HuD and IMP1, restoresGAP43mRNA and protein levels in growth cones and rescues axon outgrowth defects in SMA neurons. These findings demonstrate that SMN plays an important role in the localization and local translation of mRNAs with important axonal functions and suggest that disruption of this function may contribute to the axonal defects observed in SMA.

SIGNIFICANCE STATEMENT

The motor neuron disease spinal muscular atrophy (SMA) is caused by reduced levels of the survival of motor neuron (SMN) protein, which plays a key role in assembling RNA/protein complexes that are essential for mRNA splicing. It remains unclear whether defects in this well characterized housekeeping function cause the specific degeneration of spinal motor neurons observed in SMA. Here, we describe an additional role of SMN in regulating the axonal localization and local translation of the mRNA encoding growth-associated protein 43 (GAP43). This study supports a model whereby SMN deficiency impedes transport and local translation of mRNAs important for neurite outgrowth and stabilization, thus contributing to axon degeneration, muscle denervation, and motor neuron cell death in SMA.

Synaptic Failure: Focus in an Integrative View of ALS

From early description by Charcot, the classification of the Amyotrophic Lateral Sclerosis (ALS) is evolving from a subtype of Motor Neuron (MN) Disease to be considered rather a multi-systemic, non-cell autonomous and complex neurodegenerative disease. In the last decade, the huge amount of knowledge acquired has shed new insights on the pathological mechanisms underlying ALS from different perspectives. However, a whole vision on the multiple dysfunctional pathways is needed with the inclusion of information often excluded in other published revisions. We propose an integrative view of ALS pathology, although centered on the synaptic failure as a converging and crucial player to the etiology of the disease. Homeostasis of input and output synaptic activity of MNs has been proved to be severely and early disrupted and to definitively contribute to microcircuitry alterations at the spinal cord. Several cells play roles in synaptic communication across the MNs network system such as interneurons, astrocytes, microglia, Schwann and skeletal muscle cells. Microglia are described as highly dynamic surveying cells of the nervous system but also as determinant contributors to the synaptic plasticity linked to neuronal activity. Several signaling axis such as TNFα/TNFR1 and CX3CR1/CX3CL1 that characterize MN-microglia cross talk contribute to synaptic scaling and maintenance, have been found altered in ALS. The presence of dystrophic and atypical microglia in late stages of ALS, with a decline in their dynamic motility and phagocytic ability, together with less synaptic and neuronal contacts disrupts the MN-microglia dialogue, decreases homeostatic regulation of neuronal activity, perturbs “on/off” signals and accelerates disease progression associated to impaired synaptic function and regeneration. Other hotspot in the ALS affected network system is the unstable neuromuscular junction (NMJ) leading to distal axonal degeneration. Reduced neuromuscular spontaneous synaptic activity in ALS mice models was also suggested to account for the selective vulnerability of MNs and decreased regenerative capability. Synaptic destabilization may as well derive from increased release of molecules by muscle cells (e.g. NogoA) and by terminal Schwann cells (e.g. semaphorin 3A) conceivably causing nerve terminal retraction and denervation, as well as inhibition of re-connection to muscle fibers. Indeed, we have overviewed the alterations on the metabolic pathways and self-regenerative capacity presented in skeletal muscle cells that contribute to muscle wasting in ALS. Finally, a detailed footpath of pathologic changes on MNs and associated dysfunctional and synaptic alterations is provided. The oriented motivation in future ALS studies as outlined in the present article will help in fruitful novel achievements on the mechanisms involved and in developing more target-driven therapies that will bring new hope in halting or delaying disease progression in ALS patients.

Sarcopenia--The search for emerging biomarkers

Sarcopenia, an age-related decline in skeletal muscle mass and function, dramatically affects the life quality of elder people. In view of increasing life expectancy, sarcopenia renders a heavy burden on the health care system. However, although there is a consensus that sarcopenia is a multifactorial syndrome, its etiology, underlying mechanisms, and even definition remain poorly delineated, thus, preventing development of a precise treatment strategy. The main aim of our review is to critically analyze potential sarcopenia biomarkers in light of the molecular mechanisms of their involvement in sarcopenia pathogenesis. Normal muscle mass and function maintenance are proposed to be dependent on the dynamic balance between the positive regulators of muscle growth such as bone morphogenetic proteins (BMPs), brain-derived neurotrophic factor (BDNF), follistatin (FST) and irisin, and negative regulators including TGFβ, myostatin, activins A and B, and growth and differentiation factor-15 (GDF-15). We hypothesize that the shift in this balance to muscle growth inhibitors, along with increased expression of the C- terminal agrin fragment (CAF) associated with age-dependent neuromuscular junction (NMJ) dysfunction, as well as skeletal muscle-specific troponin T (sTnT), a key component of contractile machinery, is a main mechanism underlying sarcopenia pathogenesis. Thus, this review proposes and emphasizes that these molecules are the emerging sarcopenia biomarkers.

Effects of electroacupuncture on recovery of the electrophysiological properties of the rabbit gastrocnemius after contusion: an in vivo animal study

Background

Our preliminary studies indicated that electroacupuncture (EA) at the ST36 and Ashi acupoints could promote regeneration of the rabbit gastrocnemius (GM) by improving microcirculation perfusion, promoting the recovery of myofiber structures, and inhibiting excessive fibrosis. However, the effects of EA on recovery of the electrophysiological properties of the GM after contusion are not yet clear. Thus, the purpose of this study was to investigate the effects of EA at the Zusanli (ST36) and Ashi acupoints with regard to recovery of the electrophysiological properties of the rabbit GM after contusion.

Methods

Forty-five rabbits were randomly divided into three groups: normal, contusion, and EA. After an acute GM contusion was produced (in rabbits in the contusion and EA groups), rabbits in the EA group were treated with electrostimulation at the ST36 and Ashi acupoints with 0.4 mA (2 Hz) for 15 min. The contusion group received no EA treatment. At different time points (7, 14, and 28 days) after contusion, we performed surface electromyography (EMG) and measured the nerve conduction velocity (NCV) of the GM and the GM branch of the tibial nerve. We also examined acetylcholinesterase (AchE) and Agrin expression in the neuromuscular junction (NMJ) via immunohistochemistry.

Results

Compared with the contusion group, the EMG amplitude and NCV in rabbits in the EA group were significantly higher at all time points after contusion. AchE and Agrin expression in the EA group were significantly higher than those in the contusion group.

Conclusions

Our results showed that EA at the ST36 and Ashi acupoints effectively promoted recovery of the electrophysiological properties of the rabbit GM after contusion. The effects of EA were realized by promotion of the regeneration of myofibers and nerve fibers, as well as acceleration of NMJ reconstruction by upregulation of AchE and Agrin expression in the motor endplate area.

Transcriptional profile of muscle following acute induction of symptoms in a mouse model of Kennedy's disease/spinobulbar muscular atrophy

BACKGROUND

Kennedy's disease/Spinobulbar muscular atrophy (KD/SBMA) is a degenerative neuromuscular disease affecting males. This disease is caused by polyglutamine expansion mutations of the androgen receptor (AR) gene. Although KD/SBMA has been traditionally considered a motor neuron disease, emerging evidence points to a central etiological role of muscle. We previously reported a microarray study of genes differentially expressed in muscle of three genetically unique mouse models of KD/SBMA but were unable to detect those which are androgen-dependent or are associated with onset of symptoms.

METHODOLOGY/PRINCIPAL FINDINGS

In the current study we examined the time course and androgen-dependence of transcriptional changes in the HSA-AR transgenic (Tg) mouse model, in which females have a severe phenotype after acute testosterone treatment. Using microarray analysis we identified differentially expressed genes at the onset and peak of muscle weakness in testosterone-treated Tg females. We found both transient and persistent groups of differentially expressed genes and analysis of gene function indicated functional groups such as mitochondrion, ion and nucleotide binding, muscle development, and sarcomere maintenance.

CONCLUSIONS/SIGNIFICANCE

By comparing the current results with those from the three previously reported models we were able to identify KD/SBMA candidate genes that are androgen dependent, and occur early in the disease process, properties which are promising for targeted therapeutics.

The Hippo pathway effector YAP is a critical regulator of skeletal muscle fibre size

The Yes-associated protein (YAP) is a core effector of the Hippo pathway, which regulates proliferation and apoptosis in organ development. YAP function has been extensively characterized in epithelial cells and tissues, but its function in adult skeletal muscle remains poorly defined. Here we show that YAP positively regulates basal skeletal muscle mass and protein synthesis. Mechanistically, we show that YAP regulates muscle mass via interaction with TEAD transcription factors. Furthermore, YAP abundance and activity in muscles is increased following injury or degeneration of motor nerves, as a process to mitigate neurogenic muscle atrophy. Our findings highlight an essential role for YAP as a positive regulator of skeletal muscle size. Further investigation of interventions that promote YAP activity in skeletal muscle might aid the development of therapeutics to combat muscle wasting and neuromuscular disorders.

FunMod: a Cytoscape plugin for identifying functional modules in undirected protein-protein networks

The characterization of the interacting behaviors of complex biological systems is a primary objective in protein-protein network analysis and computational biology. In this paper we present FunMod, an innovative Cytoscape version 2.8 plugin that is able to mine undirected protein-protein networks and to infer sub-networks of interacting proteins intimately correlated with relevant biological pathways. This plugin may enable the discovery of new pathways involved in diseases. In order to describe the role of each protein within the relevant biological pathways, FunMod computes and scores three topological features of the identified sub-networks. By integrating the results from biological pathway clustering and topological network analysis, FunMod proved to be useful for the data interpretation and the generation of new hypotheses in two case studies.

Alterations in muscle proteome of patients diagnosed with amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a motor neuron disease characterized by progressive muscle paralysis. Currently clinical tools for ALS diagnostics do not perform well enough and their improvement is needed. The objective of this study was to identify specific protein alterations related to the development of ALS using tiny muscle biopsies. We applied a shotgun proteomics and quantitative dimethyl labeling in order to analyze the global changes in human skeletal muscle proteome of ALS versus healthy subjects for the first time. 235 proteins were quantified and 11 proteins were found significantly regulated in ALS muscles. These proteins are involved in muscle development and contraction, metabolic processes, enzyme activity, regulation of apoptosis and transport activity. In order to eliminate a risk to confuse ALS with other denervations, muscle biopsies of patients with postpolio syndrome and Charcot-Marie-Tooth disease (negative controls) were compared to those of ALS and controls. Only few proteins significantly regulated in ALS patients compared to controls were affected differently in negative controls. These proteins (BTB and kelch domain-containing protein 10, myosin light chain 3, glycogen debranching enzyme, transitional endoplasmic reticulum ATPase), individually or as a panel, could be selected for estimation of ALS diagnosis and development.

BIOLOGICAL SIGNIFICANCE

ALS is a devastating neurodegenerative disease, and luckily, very rare: only one to two people out of 100,000 develop ALS yearly. This fact, however, makes studies of ALS very challenging since it is very difficult to collect the representative set of clinical samples and this may take up to several years. In this study we collected the muscle biopsies from 12 ALS patients and compared the ALS muscle proteome against the one from control subjects. We suggested the efficient method for such comprehensive quantitative analysis by LC-MS and performed it for the first time using human ALS material. This gel- and antibody-free method can be widely applied for muscle proteome studies and has been used by us for revealing of the specific protein alterations associated with ALS.

Injection of a soluble fragment of neural agrin (NT-1654) considerably improves the muscle pathology caused by the disassembly of the neuromuscular junction

Treatment of neuromuscular diseases is still an unsolved problem. Evidence over the last years strongly indicates the involvement of malformation and dysfunction of neuromuscular junctions in the development of such medical conditions. Stabilization of NMJs thus seems to be a promising approach to attenuate the disease progression of muscle wasting diseases. An important pathway for the formation and maintenance of NMJs is the agrin/Lrp4/MuSK pathway. Here we demonstrate that the agrin biologic NT-1654 is capable of activating the agrin/Lrp4/MuSK system in vivo, leading to an almost full reversal of the sarcopenia-like phenotype in neurotrypsin-overexpressing (SARCO) mice. We also show that injection of NT-1654 accelerates muscle re-innervation after nerve crush. This report demonstrates that a systemically administered agrin fragment has the potential to counteract the symptoms of neuromuscular disorders.

In vitro Differentiation of Functional Human Skeletal Myotubes in a Defined System

In vitro human skeletal muscle systems are valuable tools for the study of human muscular development, disease and treatment. However, published in vitro human muscle systems have so far only demonstrated limited differentiation capacities. Advanced differentiation features such as cross-striations and contractility have only been observed in co-cultures with motoneurons. Furthermore, it is commonly regarded that cultured human myotubes do not spontaneously contract, and any contraction has been considered to originate from innervation. This study developed a serum-free culture system in which human skeletal myotubes demonstrated advanced differentiation. Characterization by immunocytochemistry, electrophysiology and analysis of contractile function revealed these major features: A) well defined sarcomeric development, as demonstrated by the presence of cross-striations. B) finely developed excitation-contraction coupling apparatus characterized by the close apposition of dihydropyridine receptors on T-tubules and Ryanodine receptors on sarcoplasmic reticulum membranes. C) spontaneous and electrically controlled contractility. This report not only demonstrates an improved level of differentiation of cultured human skeletal myotubes, but also provides the first published evidence that such myotubes are capable of spontaneous contraction. Use of this functional in vitro human skeletal muscle system would advance studies concerning human skeletal muscle development and physiology, as well as muscle-related disease and therapy.

A necrotic stimulus is required to maximize matrix-mediated myogenesis in mice

Biomaterials that are similar to skeletal muscle extracellular matrix have been shown to augment regeneration in ischemic muscle. In this study, treatment with a collagen-based matrix stimulated molecular myogenesis in an mdx murine model of necrosis. Matrix-treated animals ran ≥ 40% further, demonstrating functional regeneration, and expressed increased levels of myogenic transcripts. By contrast, matrix treatment was unable to induce transcriptional or functional changes in an MLC/SOD1(G93A) atrophic mouse model. In vitro, satellite cells were cultured under standard conditions, on matrix, in the presence of myocyte debris (to simulate a necrotic-like environment) or with both matrix and necrotic stimuli. Exposure to both matrix and necrotic stimuli induced the greatest increases in mef2c, myf5, myoD and myogenin transcripts. Furthermore, conditioned medium collected from satellite cells cultured with both stimuli contained elevated levels of factors that modulate satellite cell activation and proliferation, such as FGF-2, HGF and SDF-1. Application of the conditioned medium to C2C12 myoblasts accelerated maturation, as demonstrated by increased mef2c, myf5 and myogenin transcripts and fusion indexes. In summary, the collagen matrix required a necrotic stimulus to enhance the maturation of satellite cells and their secretion of a myogenic cocktail. Considering that matrix treatment supports myogenesis only in in vivo models that exhibit necrosis, this study demonstrates that a necrotic environment is required to maximize matrix-mediated myogenesis.

Intramuscular scAAV9-SMN injection mediates widespread gene delivery to the spinal cord and decreases disease severity in SMA mice

We have recently demonstrated the remarkable efficiency of self-complementary (sc) AAV9 vectors for central nervous system (CNS) gene transfer following intravenous delivery in mice and larger animals. Here, we investigated whether gene delivery to motor neurons (MNs) could also be achieved via intramuscular (i.m.) scAAV9 injection and subsequent retrograde transport along the MNs axons. Unexpectedly, we found that a single injection of scAAV9 into the adult mouse gastrocnemius (GA) mediated widespread MN transduction along the whole spinal cord, without limitation to the MNs connected to the injected muscle. Spinal cord astrocytes and peripheral organs were also transduced, indicating vector spread from the injected muscle to both the CNS and the periphery through release into the blood circulation. Moreover, we showed that i.m. injection of scAAV9 vectors expressing "survival of motor neuron" (Smn) in spinal muscular atrophy (SMA) mice mediated high survival motor neuron (SMN) expression levels at both the CNS and the periphery, and increased the median lifespan from 12 days to 163 days. These findings represent to date the longest extent in survival obtained in SMA mice following i.m. viral vector gene delivery, and might generate a renewed interest in the use of i.m. adeno-associated viruses (AAV) delivery for the development of gene therapy strategies for MN diseases.

Disruption of skeletal muscle mitochondrial network genes and miRNAs in amyotrophic lateral sclerosis

Skeletal muscle mitochondrial dysfunction is believed to play a role in the progression and severity of amyotrophic lateral sclerosis (ALS). The regulation of transcriptional co-activators involved in mitochondrial biogenesis and function in ALS is not well known. When compared with healthy control subjects, patients with ALS, but not neurogenic disease (ND), had lower levels of skeletal muscle peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) mRNA and protein and estrogen-related receptor-α (ERRα) and mitofusin-2 (Mfn2) mRNA. PGC-1β, nuclear respiratory factor-1 (NRF-1) and Mfn1 mRNA as well as cytochrome C oxidase subunit IV (COXIV) mRNA and protein were lower in patients with ALS and ND. Both patient groups had reductions in citrate synthase and cytochrome c oxidase activity. Similar observations were made in skeletal muscle from transgenic ALS G93A transgenic mice. In vitro, PGC-1α and PGC-1β regulated Mfn1 and Mfn2 in an ERRα-dependent manner. Compared to healthy controls, miRNA 23a, 29b, 206 and 455 were increased in skeletal muscle of ALS patients. miR-23a repressed PGC-1α translation in a 3' UTR dependent manner. Transgenic mice over expressing miR-23a had a reduction in PGC-1α, cytochome-b and COXIV protein levels. These results show that skeletal muscle mitochondrial dysfunction in ALS patients is associated with a reduction in PGC-1α signalling networks involved in mitochondrial biogenesis and function, as well as increases in several miRNAs potentially implicated in skeletal muscle and neuromuscular junction regeneration. As miR-23a negatively regulates PGC-1α signalling, therapeutic inhibition of miR-23a may be a strategy to rescue PGC-1α activity and ameliorate skeletal muscle mitochondrial function in ALS.

Neuromuscular effects of G93A-SOD1 expression in zebrafish

Background

Amyotrophic lateral sclerosis (ALS) is a fatal disorder involving the degeneration and loss of motor neurons. The mechanisms of motor neuron loss in ALS are unknown and there are no effective treatments. Defects in the distal axon and at the neuromuscular junction are early events in the disease course, and zebrafish provide a promising in vivo system to examine cellular mechanisms and treatments for these events in ALS pathogenesis.

Results

We demonstrate that transient genetic manipulation of zebrafish to express G93A-SOD1, a mutation associated with familial ALS, results in early defects in motor neuron outgrowth and axonal branching. This is consistent with previous reports on motor neuron axonal defects associated with familial ALS genes following knockdown or mutant protein overexpression. We also demonstrate that upregulation of growth factor signaling is capable of rescuing these early defects, validating the potential of the model for therapeutic discovery. We generated stable transgenic zebrafish lines expressing G93A-SOD1 to further characterize the consequences of G93A-SOD1 expression on neuromuscular pathology and disease progression. Behavioral monitoring reveals evidence of motor dysfunction and decreased activity in transgenic ALS zebrafish. Examination of neuromuscular and neuronal pathology throughout the disease course reveals a loss of neuromuscular junctions and alterations in motor neuron innervations patterns with disease progression. Finally, motor neuron cell loss is evident later in the disease.

Conclusions

This sequence of events reflects the stepwise mechanisms of degeneration in ALS, and provides a novel model for mechanistic discovery and therapeutic development for neuromuscular degeneration in ALS.

Neuromuscular junction protection for the potential treatment of amyotrophic lateral sclerosis

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

New therapeutic approaches to spinal muscular atrophy

Bench to bedside progress has been widely anticipated for a growing number of neurodegenerative disorders. Of these, spinal muscular atrophy (SMA) is perhaps the best poised to capitalize on advances in targeted therapeutics development over the next few years. Several laboratories have achieved compelling success in SMA animal models using sophisticated methods for targeted delivery, repair, or increased expression of the survival motor neuron protein, SMN. The clinical community is actively collaborating to identify, develop, and validate outcome measures and biomarkers in parallel with laboratory efforts. Innovative trial design and synergistic approaches to maximize proactive care in conjunction with treatment with one or more of the promising pharmacologic and biologic therapies currently in the pipeline will maximize our chances to achieve meaningful outcomes for patients. This review highlights recent promising scientific and clinical advances bringing us ever closer to effective treatment(s) for our patients with SMA.

Hind limb muscle atrophy precedes cerebral neuronal degeneration in G93A-SOD1 mouse model of amyotrophic lateral sclerosis: a longitudinal MRI study

Amyotrophic lateral sclerosis (ALS) is a progressive, fatal, neurodegenerative disorder caused by the degeneration of motor neurons in the CNS, which results in complete paralysis of skeletal muscles. Recent experimental studies have suggested that the disease could initiate in skeletal muscle, rather than in the motor neurons. To establish the timeframe of motor neuron degeneration in relation to muscle atrophy in motor neuron disease, we have used MRI to monitor changes throughout disease in brain and skeletal muscle of G93A-SOD1 mice, a purported model of ALS. Longitudinal MRI examination of the same animals indicated that muscle volume in the G93A-SOD1 mice was significantly reduced from as early as week 8 of life, 4 weeks prior to clinical onset. Progressive muscle atrophy from week 8 onwards was confirmed by histological analysis. In contrast, brain MRI indicated that neurodegeneration occurs later in G93A-SOD1 mice, with hyperintensity MRI signals detected only at weeks 10-18. Neurodegenerative changes were observed only in the motor nuclei areas of the brainstem; MRI changes indicative of neurodegeneration were not detected in the motor cortex where first motor neurons originate, even at the late disease stage. This longitudinal MRI study establishes unequivocally that, in the experimental murine model of ALS, muscle degeneration occurs before any evidence of neurodegeneration and clinical signs, supporting the postulate that motor neuron disease can initiate from muscle damage and result from retrograde dying-back of the motor neurons.

Cytoplasmic dynein in neurodegeneration

Cytoplasmic dynein 1 (later referred to as dynein) is the major molecular motor moving cargoes such as mitochondria, organelles and proteins towards the minus end of microtubules. Dynein is involved in multiple basic cellular functions, such as mitosis, autophagy and structure of endoplasmic reticulum and Golgi, but also in neuron specific functions in particular retrograde axonal transport. Dynein is regulated by a number of protein complexes, notably by dynactin. Several studies have supported indirectly the involvement of dynein in neurodegeneration associated with Alzheimer's disease, Parkinson's disease, Huntington's disease and motor neuron diseases. First, axonal transport disruption represents a common feature occurring in neurodegenerative diseases. Second, a number of dynein-dependent processes, including autophagy or clearance of aggregation-prone proteins, are found defective in most of these diseases. Third, a number of mutant genes in various neurodegenerative diseases are involved in the regulation of dynein transport. This includes notably mutations in the P150Glued subunit of dynactin that are found in Perry syndrome and motor neuron diseases. Interestingly, gene products that are mutant in Huntington's disease, Parkinson's disease, motor neuron disease or spino-cerebellar ataxia are also involved in the regulation of dynein motor activity or of cargo binding. Despite a constellation of indirect evidence, direct links between the motor itself and neurodegeneration are few, and this might be due to the requirement of fully active dynein for development. Here, we critically review the evidence of dynein involvement in different neurodegenerative diseases and discuss potential underlying mechanisms.