Cortactin interacts with αDystrobrevin-1 and regulates murine neuromuscular junction morphology

Neuromuscular junctions transmit signals from the nervous system to skeletal muscles, triggering their contraction, and their proper organization is essential for breathing and voluntary movements. αDystrobrevin-1 is a cytoplasmic component of the dystrophin-glycoprotein complex and has pivotal functions in regulating the integrity of muscle fibers and neuromuscular junctions. Previous studies identified that αDystrobrevin-1 functions in the organization of the neuromuscular junction and that its phosphorylation in the C-terminus is required in this process. Our proteomic screen identified several putative αDystrobrevin-1 interactors recruited to the Y730 site in phosphorylated and unphosphorylated states. Amongst various actin-modulating proteins, we identified the Arp2/3 complex regulator cortactin. We showed that similarly to αDystrobrevin-1, cortactin is strongly enriched at the neuromuscular postsynaptic machinery and obtained results suggesting that these two proteins interact in cell homogenates and at the neuromuscular junctions. Analysis of synaptic morphology in cortactin knockout mice showed abnormalities in the slow-twitching soleus muscle and not in the fast-twitching tibialis anterior. However, muscle strength examination did not reveal apparent deficits in knockout animals.

Causative variants linked with limb girdle muscular dystrophy in an Iranian population: 6 novel variants

BACKGROUND

Limb-girdle muscular dystrophy (LGMD) is a non-syndromic muscular dystrophy caused by variations in the genes involved in muscle structure, function and repair. The heterogeneity in the severity, progression, age of onset, and causative genes makes next-generation sequencing (NGS) a necessary approach for the proper diagnosis of LGMD.

METHODS

In this article, 26 Iranian patients with LGMD criteria were diagnosed with disease variants in the genes encoding calpain3, dysferlin, sarcoglycans and Laminin α-2. Patients were referred to the hospital with variable distribution of muscle wasting and progressive weakness in the body. The symptoms along with biochemical and EMG tests were suggestive of LGMD; thus the genomic DNA of patients were investigated by whole-exome sequencing including flanking intronic regions. The target genes were explored for the disease-causing variants. Moreover, the consequence of the amino acid alterations on proteins' secondary structure and function was investigated for a better understanding of the pathogenicity of variants. Variants were sorted based on the genomic region, type and clinical significance.

RESULTS

In a comprehensive investigation of previous clinical records, 6 variations were determined as novel, including c.1354-2 A > T and c.3169_3172dupCGGC in DYSF, c.568 G > T in SGCD, c.7243 C > T, c.8662_8663 insT and c. 4397G > C in LAMA2. Some of the detected variants were located in functional domains and/or near to the post-translational modification sites, altering or removing highly conserved regions of amino acid sequence.

Early Developmental Changes of Muscle Acetylcholine Receptors Are Little Influenced by Dystrophin Absence in mdx Mouse

Dystrophin is a cytoskeletal protein contributing to the organization of the neuromuscular junction. In Duchenne muscular dystrophy, due to dystrophin absence, the distribution of endplate acetylcholine receptors (AChRs) becomes disorganized. It is still debated whether this is due to the absence of dystrophin or to the repeated damage/regeneration cycles typical of dystrophic muscle. We addressed this controversy studying the endplate in the first 3 postnatal weeks, when muscle damage in dystrophic (mdx) mice is minimal. By synaptic and extra-synaptic patch-clamp recordings in acutely dissociated mdx and wt muscle fibers, we recorded unitary events due to openings of AChR-channels containing the γ and ε subunit. We also examined AChR distribution at the endplate by immunofluorescence assays. No differences between wt and mdx fibers were found in the γ/ε switch, nor in the AChR organization at the endplates up to 21 postnatal days. Conversely, we detected a delayed appearance and disappearance of patches with high channel opening frequency in mdx fibers. Our data emphasize that the innervation-dependent γ/ε switch and AChR organization in the endplate are not affected by the absence of dystrophin, while extra-synaptic AChR cluster formation and disassembly could be differentially regulated in mdx mice.

Specific Dystrophins Selectively Associate with Inhibitory and Excitatory Synapses of the Mouse Cerebellum and their Loss Alters Expression of P2X7 Purinoceptors and Pro-Inflammatory Mediators

Duchenne muscular dystrophy (DMD) patients, having mutations of the DMD gene, present with a range of neuropsychiatric disorders, in addition to the quintessential muscle pathology. The neurobiological basis remains poorly understood because the contributions of different DMD gene products (dystrophins) to the different neural networks underlying such symptoms are yet to be fully characterised. While full-length dystrophin clusters in inhibitory synapses, with inhibitory neurotransmitter receptors, the precise subcellular expression of truncated DMD gene products with excitatory synapses remains unresolved. Furthermore, inflammation, involving P2X purinoceptor 7 (P2RX7) accompanies DMD muscle pathology, yet any association with brain dystrophins is yet to be established. The aim of this study was to investigate the comparative expression of different dystrophins, alongside ionotropic glutamate receptors and P2RX7s, within the cerebellar circuitry known to express different dystrophin isoforms. Immunoreactivity for truncated DMD gene products was targeted to Purkinje cell (PC) distal dendrites adjacent to, or overlapping with, signal for GluA1, GluA4, GluN2A, and GluD2 receptor subunits. P2X7R immunoreactivity was located in Bergmann glia profiles adjacent to PC-dystrophin immunoreactivity. Ablation of all DMD gene products coincided with decreased mRNA expression for Gria2, Gria3, and Grin2a and increased GluD2 immunoreactivity. Finally, dystrophin-null mice showed decreased brain mRNA expression of P2rx7 and several inflammatory mediators. The data suggest that PCs target different dystrophin isoforms to molecularly and functionally distinct populations of synapses. In contrast to muscle, dystrophinopathy in brain leads to the dampening of the local immune system.

Cardio-respiratory and phenotypic rescue of dystrophin/utrophin-deficient mice by combination therapy

Duchenne muscular dystrophy (DMD) is a systemic progressive muscular disease caused by frame-disrupting mutations in the DMD gene. Although exon-skipping antisense oligonucleotides (AOs) are clinically approved and can correct DMD, insufficient muscle delivery limits efficacy. If AO activity can be enhanced by safe dietary supplements, clinical trials for efficacy can be undertaken rapidly to benefit patients. We showed previously that intravenous glycine enhanced phosphorodiamidate morpholino oligomer (PMO) delivery to peripheral muscles in mdx mice. Here, we demonstrate that the combination of oral glycine and metformin with intravenous PMO enhances PMO activity, dystrophin restoration, extends lifespan, and improves body-wide function and phenotypic rescue of dystrophin /utrophin double knock-out (DKO) mice without any overt adverse effects. The DKO mice treated with the combination without altering the approved administration protocol of PMO show improved cardio-respiratory and behavioral functions. Metformin and glycine individually are ineffective in DMD patients, but the combination of PMO with clinically-approved oral glycine and metformin might improve the efficacy of the treatment also in DMD patients. Our data suggest that this combination therapy might be an attractive therapy for DMD and potentially other muscle diseases requiring systemic treatment with AOs.

Mechanisms of microtubule organization in differentiated animal cells

Microtubules are polarized cytoskeletal filaments that serve as tracks for intracellular transport and form a scaffold that positions organelles and other cellular components and modulates cell shape and mechanics. In animal cells, the geometry, density and directionality of microtubule networks are major determinants of cellular architecture, polarity and proliferation. In dividing cells, microtubules form bipolar spindles that pull chromosomes apart, whereas in interphase cells, microtubules are organized in a cell type-specific fashion, which strongly correlates with cell physiology. In motile cells, such as fibroblasts and immune cells, microtubules are organized as radial asters, whereas in immotile epithelial and neuronal cells and in muscles, microtubules form parallel or antiparallel arrays and cortical meshworks. Here, we review recent work addressing how the formation of such microtubule networks is driven by the plethora of microtubule regulatory proteins. These include proteins that nucleate or anchor microtubule ends at different cellular structures and those that sever or move microtubules, as well as regulators of microtubule elongation, stability, bundling or modifications. The emerging picture, although still very incomplete, shows a remarkable diversity of cell-specific mechanisms that employ conserved building blocks to adjust microtubule organization in order to facilitate different cellular functions.

Understanding the molecular basis of cardiomyopathy

Inherited cardiomyopathies are a major cause of mortality and morbidity worldwide and can be caused by mutations in a wide range of proteins located in different cellular compartments. The present review is based on Dr. Ju Chen's 2021 Robert M. Berne Distinguished Lectureship of the American Physiological Society Cardiovascular Section, in which he provided an overview of the current knowledge on the cardiomyopathy-associated proteins that have been studied in his laboratory. The review provides a general summary of the proteins in different compartments of cardiomyocytes associated with cardiomyopathies, with specific focus on the proteins that have been studied in Dr. Chen's laboratory.

Tks5 Regulates Synaptic Podosome Formation and Stabilization of the Postsynaptic Machinery at the Neuromuscular Junction

Currently, the etiology of many neuromuscular disorders remains unknown. Many of them are characterized by aberrations in the maturation of the neuromuscular junction (NMJ) postsynaptic machinery. Unfortunately, the molecular factors involved in this process are still largely unknown, which poses a great challenge for identifying potential therapeutic targets. Here, we identified Tks5 as a novel interactor of αdystrobrevin-1, which is a crucial component of the NMJ postsynaptic machinery. Tks5 has been previously shown in cancer cells to be an important regulator of actin-rich structures known as invadosomes. However, a role of this scaffold protein at a synapse has never been studied. We show that Tks5 is crucial for remodeling of the NMJ postsynaptic machinery by regulating the organization of structures similar to the invadosomes, known as synaptic podosomes. Additionally, it is involved in the maintenance of the integrity of acetylcholine receptor (AChR) clusters and regulation of their turnover. Lastly, our data indicate that these Tks5 functions may be mediated by its involvement in recruitment of actin filaments to the postsynaptic machinery. Collectively, we show for the first time that the Tks5 protein is involved in regulation of the postsynaptic machinery.

Drebrin Regulates Acetylcholine Receptor Clustering and Organization of Microtubules at the Postsynaptic Machinery

Proper muscle function depends on the neuromuscular junctions (NMJs), which mature postnatally to complex "pretzel-like" structures, allowing for effective synaptic transmission. Postsynaptic acetylcholine receptors (AChRs) at NMJs are anchored in the actin cytoskeleton and clustered by the scaffold protein rapsyn, recruiting various actin-organizing proteins. Mechanisms driving the maturation of the postsynaptic machinery and regulating rapsyn interactions with the cytoskeleton are still poorly understood. Drebrin is an actin and microtubule cross-linker essential for the functioning of the synapses in the brain, but its role at NMJs remains elusive. We used immunohistochemistry, RNA interference, drebrin inhibitor 3,5-bis-trifluoromethyl pyrazole (BTP2) and co-immunopreciptation to explore the role of this protein at the postsynaptic machinery. We identify drebrin as a postsynaptic protein colocalizing with the AChRs both in vitro and in vivo. We also show that drebrin is enriched at synaptic podosomes. Downregulation of drebrin or blocking its interaction with actin in cultured myotubes impairs the organization of AChR clusters and the cluster-associated microtubule network. Finally, we demonstrate that drebrin interacts with rapsyn and a drebrin interactor, plus-end-tracking protein EB3. Our results reveal an interplay between drebrin and cluster-stabilizing machinery involving rapsyn, actin cytoskeleton, and microtubules.

Transcriptome changes during the initiation and progression of Duchenne muscular dystrophy in Caenorhabditis elegans

Duchenne muscular dystrophy (DMD) is a lethal, X-linked disease characterized by progressive muscle degeneration. The condition is driven by nonsense and missense mutations in the dystrophin gene, leading to instability of the sarcolemma and skeletal muscle necrosis and atrophy. Resulting changes in muscle-specific gene expression that take place in dystrophin's absence remain largely uncharacterized, as they are potentially obscured by the chronic inflammation elicited by muscle damage in humans. Caenorhabditis elegans possess a mild inflammatory response that is not active in the muscle, and lack a satellite cell equivalent. This allows for the characterization of the transcriptome rearrangements affecting disease progression independently of inflammation and regeneration. In effort to better understand these dynamics, we have isolated and sequenced body muscle-specific transcriptomes from C. elegans lacking functional dystrophin at distinct stages of disease progression. We have identified an upregulation of genes involved in mitochondrial function early in disease progression, and an upregulation of genes related to muscle repair in later stages. Our results suggest that in C. elegans, dystrophin may have a signaling role early in development, and its absence may activate compensatory mechanisms that counteract muscle degradation caused by loss of dystrophin. We have also developed a temperature-based screening method for synthetic paralysis that can be used to rapidly identify genetic partners of dystrophin. Our results allow for the comprehensive identification of transcriptome changes that potentially serve as independent drivers of disease progression and may in turn allow for the identification of new therapeutic targets for the treatment of DMD.

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.

Fructose and Mannose in Inborn Errors of Metabolism and Cancer

History suggests that tasteful properties of sugar have been domesticated as far back as 8000 BCE. With origins in New Guinea, the cultivation of sugar quickly spread over centuries of conquest and trade. The product, which quickly integrated into common foods and onto kitchen tables, is sucrose, which is made up of glucose and fructose dimers. While sugar is commonly associated with flavor, there is a myriad of biochemical properties that explain how sugars as biological molecules function in physiological contexts. Substantial research and reviews have been done on the role of glucose in disease. This review aims to describe the role of its isomers, fructose and mannose, in the context of inborn errors of metabolism and other metabolic diseases, such as cancer. While structurally similar, fructose and mannose give rise to very differing biochemical properties and understanding these differences will guide the development of more effective therapies for metabolic disease. We will discuss pathophysiology linked to perturbations in fructose and mannose metabolism, diagnostic tools, and treatment options of the diseases.

Proteomic analysis identifies key differences in the cardiac interactomes of dystrophin and micro-dystrophin

ΔR4-R23/ΔCT micro-dystrophin (μDys) is a miniaturized version of dystrophin currently evaluated in a Duchenne muscular dystrophy (DMD) gene therapy trial to treat skeletal and cardiac muscle disease. In pre-clinical studies, μDys efficiently rescues cardiac histopathology, but only partially normalizes cardiac function. To gain insights into factors that may impact the cardiac therapeutic efficacy of μDys, we compared by mass spectrometry the composition of purified dystrophin and μDys protein complexes in the mouse heart. We report that compared to dystrophin, μDys has altered associations with α1- and β2-syntrophins, as well as cavins, a group of caveolae-associated signaling proteins. In particular, we found that membrane localization of cavin-1 and cavin-4 in cardiomyocytes requires dystrophin and is profoundly disrupted in the heart of mdx^5cv mice, a model of DMD. Following cardiac stress/damage, membrane-associated cavin-4 recruits the signaling molecule ERK to caveolae, which activates key cardio-protective responses. Evaluation of ERK signaling revealed a profound inhibition, below physiological baseline, in the mdx^5cv mouse heart. Expression of μDys in mdx^5cv mice prevented the development of cardiac histopathology but did not rescue membrane localization of cavins nor did it normalize ERK signaling. Our study provides the first comparative analysis of purified protein complexes assembled in vivo by full-length dystrophin and a therapeutic micro-dystrophin construct. This has revealed disruptions in cavins and ERK signaling that may contribute to DMD cardiomyopathy. This new knowledge is important for ongoing efforts to prevent and treat heart disease in DMD patients.

Therapeutic aspects of cell signaling and communication in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a devastating chromosome X-linked disease that manifests predominantly in progressive skeletal muscle wasting and dysfunctions in the heart and diaphragm. Approximately 1/5000 boys and 1/50,000,000 girls suffer from DMD, and to date, the disease is incurable and leads to premature death. This phenotypic severity is due to mutations in the DMD gene, which result in the absence of functional dystrophin protein. Initially, dystrophin was thought to be a force transducer; however, it is now considered an essential component of the dystrophin-associated protein complex (DAPC), viewed as a multicomponent mechanical scaffold and a signal transduction hub. Modulating signal pathway activation or gene expression through epigenetic modifications has emerged at the forefront of therapeutic approaches as either an adjunct or stand-alone strategy. In this review, we propose a broader perspective by considering DMD to be a disease that affects myofibers and muscle stem (satellite) cells, as well as a disorder in which abrogated communication between different cell types occurs. We believe that by taking this systemic view, we can achieve safe and holistic treatments that can restore correct signal transmission and gene expression in diseased DMD tissues.

The Dystrophin Node as Integrator of Cytoskeletal Organization, Lateral Force Transmission, Fiber Stability and Cellular Signaling in Skeletal Muscle

The systematic bioanalytical characterization of the protein product of the DMD gene, which is defective in the pediatric disorder Duchenne muscular dystrophy, led to the discovery of the membrane cytoskeletal protein dystrophin. Its full-length muscle isoform Dp427-M is tightly linked to a sarcolemma-associated complex consisting of dystroglycans, sarcoglyans, sarcospan, dystrobrevins and syntrophins. Besides these core members of the dystrophin-glycoprotein complex, the wider dystrophin-associated network includes key proteins belonging to the intracellular cytoskeleton and microtubular assembly, the basal lamina and extracellular matrix, various plasma membrane proteins and cytosolic components. Here, we review the central role of the dystrophin complex as a master node in muscle fibers that integrates cytoskeletal organization and cellular signaling at the muscle periphery, as well as providing sarcolemmal stabilization and contractile force transmission to the extracellular region. The combination of optimized tissue extraction, subcellular fractionation, advanced protein co-purification strategies, immunoprecipitation, liquid chromatography and two-dimensional gel electrophoresis with modern mass spectrometry-based proteomics has confirmed the composition of the core dystrophin complex at the sarcolemma membrane. Importantly, these biochemical and mass spectrometric surveys have identified additional members of the wider dystrophin network including biglycan, cavin, synemin, desmoglein, tubulin, plakoglobin, cytokeratin and a variety of signaling proteins and ion channels.

AChRs Degeneration at NMJ in Aging-Associated Sarcopenia-A Systematic Review

Sarcopenia is an aging process with a decline of skeletal muscle mass and function, which is a challenging public health problem with reduced quality of life in patients. The endplate, the post-synaptic part of the neuromuscular junction (NMJ), occupies 0.1% of the myofiber surface area only, but is composed of millions of acetylcholine receptors (AChRs) that are efficient in binding to acetylcholine (ACh) and triggering skeletal muscle contraction. This systematic review aims to examine aging-associated alterations of post-synaptic AChRs, including morphology, function and related gene expression. A systematic literature search was conducted in PubMed, Embase and Web of Science with relevant keywords by two independent reviewers. Original pre-clinical and clinical studies regarding AChRs changes during aging with available full text and written in English were included. Information was extracted from the included studies for further review. In total, 30 articles were included. Various parameters assessing AChRs alterations by radioassay, immunofluorescence, electrophysiology and mechanical test were reported. Endplate fragmentation and denervation were common in old skeletal muscles during aging. To ensure efficient NMJ transmission and force generation, type I or IIb muscle fibers tended to have increased ACh quanta releasing after electrical stimulations, while type IIa muscle fibers tended to have stronger binding between ACh and AChRs, but the overall function of AChRs was reduced during aging. Alterations of AChRs area depended on muscle type, species and the progress of muscle atrophy and type I muscles fibers tended to demonstrate enlarging AChRs areas. Myogenic regulator factors (MRFs) can regulate the expression of AChRs subunits, while decreased MRF4 may lead to expression changes of AChRs subunits during aging. Sarcoglycan-α can delay low-density lipoprotein receptor-related protein 4 (LRP4) degradation. This protein was increased in old muscles but still cannot suppress the degradation of LRP4. Investigating the role of these AChRs-related genes in the process of aging may provide a potential target to treat sarcopenia.

Genetic cause of heterogeneous inherited myopathies in a cohort of Greek patients

Inherited muscle disorders are caused by pathogenic changes in numerous genes. Herein, we aimed to investigate the etiology of muscle disease in 24 consecutive Greek patients with myopathy suspected to be genetic in origin, based on clinical presentation and laboratory and electrophysiological findings and absence of known acquired causes of myopathy. Of these, 16 patients (8 females, median 24 years-old, range 7 to 67 years-old) were diagnosed by Whole Exome Sequencing as suffering from a specific type of inherited muscle disorder. Specifically, we have identified causative variants in 6 limb-girdle muscular dystrophy genes (6 patients; ANO5, CAPN3, DYSF, ISPD, LAMA2, SGCA), 3 metabolic myopathy genes (4 patients; CPT2, ETFDH, GAA), 1 congenital myotonia gene (1 patient; CLCN1), 1 mitochondrial myopathy gene (1 patient; MT-TE) and 3 other myopathy-associated genes (4 patients; CAV3, LMNA, MYOT). In 6 additional family members affected by myopathy, we reached genetic diagnosis following identification of a causative variant in an index patient. In our patients, genetic diagnosis ended a lengthy diagnostic process and, in the case of Multiple acyl-CoA dehydrogenase deficiency and Pompe's disease, it enabled specific treatment to be initiated. These results further expand the genotypic and phenotypic spectrum of inherited myopathies.

A Role for Caveolin-3 in the Pathogenesis of Muscular Dystrophies

Caveolae are the cholesterol-rich small invaginations of the plasma membrane present in many cell types including adipocytes, endothelial cells, epithelial cells, fibroblasts, smooth muscles, skeletal muscles and cardiac muscles. They serve as specialized platforms for many signaling molecules and regulate important cellular processes like energy metabolism, lipid metabolism, mitochondria homeostasis, and mechano-transduction. Caveolae can be internalized together with associated cargo. The caveolae-dependent endocytic pathway plays a role in the withdrawal of many plasma membrane components that can be sent for degradation or recycled back to the cell surface. Caveolae are formed by oligomerization of caveolin proteins. Caveolin-3 is a muscle-specific isoform, whose malfunction is associated with several diseases including diabetes, cancer, atherosclerosis, and cardiovascular diseases. Mutations in Caveolin-3 are known to cause muscular dystrophies that are collectively called caveolinopathies. Altered expression of Caveolin-3 is also observed in Duchenne’s muscular dystrophy, which is likely a part of the pathological process leading to muscle weakness. This review summarizes the major functions of Caveolin-3 in skeletal muscles and discusses its involvement in the pathology of muscular dystrophies.

Astrocyte responses to experimental glaucoma in mouse optic nerve head

PURPOSE

To delineate responses of optic nerve head astrocytes to sustained intraocular pressure (IOP) elevation in mice.

METHODS

We elevated IOP for 1 day to 6 weeks by intracameral microbead injection in 4 strains of mice. Astrocyte alterations were studied by transmission electron microscopy (TEM) including immunogold molecular localization, and by laser scanning microscopy (LSM) with immunofluorescence for integrin β1, α-dystroglycan, and glial fibrillary acidic protein (GFAP). Astrocyte proliferation and apoptosis were quantified by Ki67 and TUNEL labeling, respectively.

RESULTS

Astrocytes in normal optic nerve head expressed integrin β1 and α-dystroglycan by LSM and TEM immunogold labeling at electron dense junctional complexes that were found only on cell membrane zones bordering their basement membranes (BM) at the peripapillary sclera (PPS) and optic nerve head capillaries. At 1-3 days after IOP elevation, abnormal extracellular spaces appeared between astrocytes near PPS, and axonal vesical and mitochondrial accumulation indicated axonal transport blockade. By 1 week, abnormal spaces increased, new collagen formation occurred, and astrocytes separated from their BM, leaving cell membrane fragments. Electron dense junctional complexes separated or were absent at the BM. Astrocyte proliferation was modest during the first week, while only occasional apoptotic astrocytes were observed by TEM and TUNEL.

CONCLUSIONS

Astrocytes normally exhibit junctions with their BM which are disrupted by extended IOP elevation. Responses include reorientation of cell processes, new collagen formation, and cell proliferation.

Defects of full-length dystrophin trigger retinal neuron damage and synapse alterations by disrupting functional autophagy

Dystrophin (dys) mutations predispose Duchenne muscular disease (DMD) patients to brain and retinal complications. Although different dys variants, including long dys products, are expressed in the retina, their function is largely unknown. We investigated the putative role of full-length dystrophin in the homeostasis of neuro-retina and its impact on synapsis stabilization and cell fate. Retinas of mdx mice, the most used DMD model which does not express the 427-KDa dys protein (Dp427), showed overlapped cell death and impaired autophagy. Apoptotic neurons in the outer plexiform/inner nuclear layer and the ganglion cell layer had an impaired autophagy with accumulated autophagosomes. The autophagy dysfunction localized at photoreceptor axonal terminals and bipolar, amacrine, and ganglion cells. The absence of Dp427 does not cause a severe phenotype but alters the neuronal architecture, compromising mainly the pre-synaptic photoreceptor terminals and their post-synaptic sites. The analysis of two dystrophic mutants of the fruit fly Drosophila melanogaster, the homozygous Dys^E17 and Dys^EP3397, lacking functional large-isoforms of dystrophin-like protein, revealed rhabdomere degeneration. Structural damages were evident in the internal network of retina/lamina where photoreceptors make the first synapse. Both accumulated autophagosomes and apoptotic features were detected and the visual system was functionally impaired. The reactivation of the autophagosome turnover by rapamycin prevented neuronal cell death and structural changes of mutant flies and, of interest, sustained autophagy ameliorated their response to light. Overall, these findings indicate that functional full-length dystrophin is required for synapsis stabilization and neuronal survival of the retina, allowing also proper autophagy as a prerequisite for physiological cell fate and visual properties.

Arhgef5 Binds α-Dystrobrevin 1 and Regulates Neuromuscular Junction Integrity

The neuromuscular junctions (NMJs) connect muscle fibers with motor neurons and enable the coordinated contraction of skeletal muscles. The dystrophin-associated glycoprotein complex (DGC) is an essential component of the postsynaptic machinery of the NMJ and is important for the maintenance of NMJ structural integrity. To identify novel proteins that are important for NMJ organization, we performed a mass spectrometry-based screen for interactors of α-dystrobrevin 1 (aDB1), one of the components of the DGC. The guanidine nucleotide exchange factor (GEF) Arhgef5 was found to be one of the aDB1 binding partners that is recruited to Tyr-713 in a phospho-dependent manner. We show here that Arhgef5 localizes to the NMJ and that its genetic depletion in the muscle causes the fragmentation of the synapses in conditional knockout mice. Arhgef5 loss in vivo is associated with a reduction in the levels of active GTP-bound RhoA and Cdc42 GTPases, highlighting the importance of actin dynamics regulation for the maintenance of NMJ integrity.

Multiple MuSK signaling pathways and the aging neuromuscular junction

The neuromuscular junction (NMJ) is the vehicle for fast, reliable and robust communication between motor neuron and muscle. The unparalleled accessibility of this synapse to morphological, electrophysiological and genetic analysis has yielded an in depth understanding of many molecular components mediating its formation, maturation and stability. However, key questions surrounding the signaling pathways mediating these events and how they play out across the lifetime of the synapse remain unanswered. Such information is critical since the NMJ is necessary for normal movement and is compromised in several settings including myasthenia gravis, amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), muscular dystrophy, sarcopenia and aging. Muscle specific kinase (MuSK) is a central player in most if not all contexts of NMJ formation and stability. However, elucidating the function of this receptor in this range of settings is challenging since MuSK participates in at least three signaling pathways: as a tyrosine kinase-dependent receptor for agrin-LRP4 and Wnts; and, as a kinase-independent BMP co-receptor. Here we focus on NMJ stability during aging and discuss open questions regarding the molecular mechanisms that govern active maintenance of the NMJ, with emphasis on MuSK and the potential role of its multiple signaling contexts.

An improved method for culturing myotubes on laminins for the robust clustering of postsynaptic machinery

Motor neurons form specialized synapses with skeletal muscle fibers, called neuromuscular junctions (NMJs). Cultured myotubes are used as a simplified in vitro system to study the postsynaptic specialization of muscles. The stimulation of myotubes with the glycoprotein agrin or laminin-111 induces the clustering of postsynaptic machinery that contains acetylcholine receptors (AChRs). When myotubes are grown on laminin-coated surfaces, AChR clusters undergo developmental remodeling to form topologically complex structures that resemble mature NMJs. Needing further exploration are the molecular processes that govern AChR cluster assembly and its developmental maturation. Here, we describe an improved protocol for culturing muscle cells to promote the formation of complex AChR clusters. We screened various laminin isoforms and showed that laminin-221 was the most potent for inducing AChR clusters, whereas laminin-121, laminin-211, and laminin-221 afforded the highest percentages of topologically complex assemblies. Human primary myotubes that were formed by myoblasts obtained from patient biopsies also assembled AChR clusters that underwent remodeling in vitro. Collectively, these results demonstrate an advancement of culturing myotubes that can facilitate high-throughput screening for potential therapeutic targets for neuromuscular disorders.

Proteomic profiling of fatty acid binding proteins in muscular dystrophy

Introduction: Duchenne muscular dystrophy is a neuromuscular disorder, which is caused by abnormalities in the DMD gene that encodes the membrane cytoskeletal protein dystrophin. Besides progressive skeletal muscle wasting, dystrophinopathy also affects non-skeletal muscle tissues, including cells in the cardio-respiratory system, the central nervous system, the liver and the kidney.Areas covered: This review summarizes the proteomic characterization of a key class of lipid chaperones, the large family of fatty acid binding proteins, and their potential role in muscular dystrophy. Recent proteomic surveys using animal models and patient specimens are reviewed. Pathobiochemical changes in specific proteoforms of fatty acid binding protein in the multi-system pathology of dystrophinopathy are discussed.Expert opinion: The mass spectrometric identification of distinct changes in fatty acid binding proteins in muscle, heart, liver, kidney and serum demonstrates that considerable alterations occur in key steps of metabolite transport and fat metabolism in muscular dystrophy. These new findings might be helpful to further develop a comprehensive biomarker signature of metabolic changes in X-linked muscular dystrophy, which should improve (i) our understanding of complex pathobiochemical changes due to dystrophin deficiency, (ii) the identification of novel therapeutic targets, and (iii) the design of differential diagnostic, prognostic and therapy-monitoring approaches.

The Effect of Inspiratory Muscle Training on Duchenne Muscular Dystrophy: A Meta-analysis

PURPOSE

This study reports the respiratory muscle training effect on strength and endurance in individuals with Duchenne muscular dystrophy.

METHODS

Articles published from 1984 to 2017 were reviewed. Six articles met the inclusion criteria that included within-subject control or between-subject control group, participants with a diagnosis of only Duchenne muscular dystrophy, participation in respiratory muscle training intervention, and outcome measures of endurance and strength. Effect sizes were calculated for each study and overall, weighted mean effect sizes for strength and endurance outcome measures.

RESULTS

There was a large effect for improving respiratory endurance and a moderate effect for muscle strength. However, these effects were not significant.

CONCLUSION

Findings justify further exploration of the potential benefits of respiratory muscle training for individuals with Duchenne muscular dystrophy.

Decoding WW domain tandem-mediated target recognitions in tissue growth and cell polarity

WW domain tandem-containing proteins such as KIBRA, YAP, and MAGI play critical roles in cell growth and polarity via binding to and positioning target proteins in specific subcellular regions. An immense disparity exists between promiscuity of WW domain-mediated target bindings and specific roles of WW domain proteins in cell growth regulation. Here, we discovered that WW domain tandems of KIBRA and MAGI, but not YAP, bind to specific target proteins with extremely high affinity and exquisite sequence specificity. Via systematic structural biology and biochemistry approaches, we decoded the target binding rules of WW domain tandems from cell growth regulatory proteins and uncovered a list of previously unknown WW tandem binding proteins including β-Dystroglycan, JCAD, and PTPN21. The WW tandem-mediated target recognition mechanisms elucidated here can guide functional studies of WW domain proteins in cell growth and polarity as well as in other cellular processes including neuronal synaptic signaling.

miR-146a deficiency does not aggravate muscular dystrophy in mdx mice

Duchenne muscular dystrophy (DMD) is a genetic disease evoked by a mutation in the dystrophin gene. It is associated with progressive muscle degeneration and increased inflammation. Up to this date, mainly anti-inflammatory treatment is available for patients suffering from DMD. miR-146a is known to diminish inflammation and fibrosis in different tissues by downregulating the expression of proinflammatory cytokines. However, its role in DMD has not been studied so far.

In our work, we have generated mice globally lacking both dystrophin and miR-146a (miR-146a^−/−mdx) and examined them together with wild-type, single miR-146a knockout and dystrophic (mdx—lacking dystrophin) mice in a variety of aspects associated with DMD pathophysiology (muscle degeneration, inflammatory reaction, muscle satellite cells, muscle regeneration, and fibrosis).

We have shown that miR-146a level is increased in dystrophic muscles in comparison to wild-type mice. Its deficiency augments the expression of proinflammatory cytokines (IL-1β, CCL2, TNFα). However, muscle degeneration was not significantly worsened in mdx mice lacking miR-146a up to 24 weeks of age, although some aggravation of muscle damage and inflammation was evident in 12-week-old animals, though no effect of miR-146a deficiency was visible on quantity, proliferation, and in vitro differentiation of muscle satellite cells isolated from miR-146a^−/−mdx mice vs. mdx. Similarly, muscle regeneration and collagen deposition were not changed by miR-146a deficiency. Nevertheless, the lack of miR-146a is associated with decreased Vegfa and increased Tgfb1.

Overall, the lack of miR-146a did not aggravate significantly the dystrophic conditions in mdx mice, but its effect on DMD in more severe conditions warrants further investigation.

CBP and P300 regulate distinct gene networks required for human primary myoblast differentiation and muscle integrity

The acetyltransferases CBP and P300 have been implicated in myogenesis in mouse immortalized cell lines but these studies focused only on the expression of a handful of myogenic factors. Hence, the respective role of these two related cofactors and their impact at global scale on gene expression rewiring during primary myoblast differentiation remain unknown. Here, we characterised the gene networks regulated by these two epigenetic enzymes during human primary myoblast differentiation (HPM). We found that CBP and p300 play a critical role in the activation of the myogenic program and mostly regulate distinct gene sets to control several aspects of HPM biology, even though they also exhibit some degree of redundancy. Moreover, CBP or P300 knockdown strongly impaired muscle cell adhesion and resulted in the activation of inflammation markers, two hallmarks of dystrophic disease. This was further validated in zebrafish where inhibition of CBP and P300 enzymatic activities led to cell adhesion defects and muscle fiber detachment. Our data highlight an unforeseen link between CBP/P300 activity and the emergence of dystrophic phenotypes. They thereby identify CBP and P300 as mediators of adult muscle integrity and suggest a new lead for intervention in muscular dystrophy.

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.

Perfect chronic skeletal muscle regeneration in adult spiny mice, Acomys cahirinus

The spiny mouse, Acomys cahirinus, is an adult mammal capable of remarkable feats of scar-free tissue regeneration after damage to several organs including the skin and the heart. Here we investigate the regenerative properties of the skeletal muscle of A. cahirinus tibialis anterior in comparison to the lab mouse, Mus musculus. The A. cahirinus TA showed a similar distribution of myosin heavy chain fibre types and a reduced proportion of oxidative fibres compared to M. musculus. There were differences in the matrix components of the TA with regard to collagen VI and the biomechanical properties. A. cahirinus TA regenerated faster with a more rapid induction of embryonic myosin and higher levels of dystrophin than in M. musculus fibres. There were lower levels of inflammation (NF-kB), fibrosis (TGFβ-1, collagens) and higher levels of the anti-inflammatory cytokine Cxcl12. There was a difference in macrophage profile between the two species. After multiple rounds of muscle regeneration the M. musculus TA failed to regenerate muscle fibres and instead produced a large numbers of adipocytes whereas the A. cahirinus TA regenerated perfectly. This clearly improved regeneration performance can be explained by differing levels of growth factors such as adiponectin between the two species.

Comparative gel-based proteomic analysis of chemically crosslinked complexes in dystrophic skeletal muscle

Duchenne muscular dystrophy is a highly progressive muscle wasting disease with a complex pathophysiology that is based on primary abnormalities in the dystrophin gene. In order to study potential changes in the oligomerization of high-molecular-mass protein complexes in dystrophic skeletal muscle, chemical crosslinking was combined with mass spectrometric analysis. The biochemical stabilization of protein interactions was carried out with the homo-bifunctional and amine-reactive agent bis[sulfosuccinimidyl]suberate, followed by protein shift analysis in one-dimensional gels. The proteomic approach identified 11 and 15 protein species in wild type versus dystrophic microsomal fractions, respectively, as well as eight common proteins, with an electrophoretic mobility shift to very high molecular mass following chemical crosslinking. In dystrophin-deficient preparations, several protein species with an increased tendency of oligomerisation were identified as components of the sarcolemma and its associated intra- and extracellular structures, as well as mitochondria. This included the sarcolemmal proteins myoferlin and caveolin, the cytoskeletal components vimentin and tubulin, extracellular collagen alpha-1(XII) and the mitochondrial trifunctional enzyme and oxoglutarate dehydrogenase. These changes are probably related to structural and metabolic adaptations, especially cellular repair processes, which agrees with the increased oligomerisation of myosin-3, myosin-9 and actin, and their role in cellular regeneration and structural adjustments in dystrophinopathy.

PTPα is required for laminin-2-induced Fyn-Akt signaling to drive oligodendrocyte differentiation

Extrinsic signals that regulate oligodendrocyte maturation and subsequent myelination are essential for central nervous system development and regeneration. Deficiency in the extracellular factor laminin-2 (Lm2), as occurs in congenital muscular dystrophy, can lead to impaired oligodendroglial development and aberrant myelination, but many aspects of Lm2-regulated oligodendroglial signaling and differentiation remain undefined. We show that receptor-like protein tyrosine phosphatase alpha (PTPα) is essential for myelin basic protein expression and cell spreading during Lm2-induced oligodendrocyte differentiation. PTPα complexes with the Lm2 receptors α6β1 integrin and dystroglycan to transduce Fyn activation upon Lm2 engagement. In this way, PTPα mediates a subset of Lm2-induced signals required for differentiation that includes mTOR-dependent Akt activation but not Erk activation. We identify N-myc downstream regulated gene-1 (NDRG1) as a PTPα-regulated molecule during oligodendrocyte differentiation and distinguish Lm2 receptor-specific modes of Fyn-Akt-dependent and -independent NDRG1 phosphorylation. Altogether, this reveals a Lm2-regulated PTPα-Fyn-Akt signaling axis that is critical for key aspects of the gene expression and morphological changes that mark oligodendrocyte maturation.

Myofibers

Muscle tissue is a highly specialized type of tissue, made up of cells that have as their fundamental properties excitability and contractility. The cellular elements that make up this type of tissue are called muscle fibers, or myofibers, because of the elongated shape they have. Contractility is due to the presence of myofibrils in the muscle fiber cytoplasm, as large cellular assemblies. Also, myofibers are responsible for the force that the muscle generates which represents a countless aspect of human life. Movements due to muscles are based on the ability of muscle fibers to use the chemical energy procured in metabolic processes, to shorten and then to return to the original dimensions. We describe in detail the levels of organization for the myofiber, and we correlate the structural aspects with the functional ones, beginning with neuromuscular transmission down to the biochemical reactions achieved in the sarcoplasmic reticulum by the release of Ca²⁺ and the cycling of crossbridges. Furthermore, we are reviewing the types of muscle contractions and the fiber-type classification.