Removal of dystroglycan causes severe muscular dystrophy in zebrafish embryos

Muscular dystrophy is frequently caused by disruption of the dystrophin-glycoprotein complex (DGC), which links muscle cells to the extracellular matrix. Dystroglycan, a central component of the DGC, serves as a laminin receptor via its extracellular alpha subunit, and interacts with dystrophin (and thus the actin cytoskeleton) through its integral membrane beta subunit. We have removed the function of dystroglycan in zebrafish embryos. In contrast to mouse, where dystroglycan mutations lead to peri-implantation lethality, dystroglycan is dispensable for basement membrane formation during early zebrafish development. At later stages, however, loss of dystroglycan leads to a disruption of the DGC, concurrent with loss of muscle integrity and necrosis. In addition, we find that loss of the DGC leads to loss of sarcomere and sarcoplasmic reticulum organisation. The DGC is required for long-term survival of muscle cells in zebrafish, but is dispensable for muscle formation. Dystroglycan or the DGC is also required for normal sarcomere and sarcoplasmic reticulum organisation. Because zebrafish embryos lacking dystroglycan share several characteristics with human muscular dystrophy, they should serve as a useful model for the disease. In addition, knowing the dystroglycan null phenotype in zebrafish will facilitate the isolation of other molecules involved in muscular dystrophy pathogenesis.     

Dystrophin in adult zebrafish muscle

Mutations in the human dystrophin gene are implicated in the fatal muscle wasting disease Duchenne Muscular Dystrophy (DMD). This gene expresses a sarcolemmal-associated protein that is evolutionarily conserved, underpinning its important role in the architecture of muscle. In terms of DMD modelling, the mouse has served as a suitable vertebrate species but the pathophysiology of the disease in the mouse does not entirely mimic human DMD. We have examined the zebrafish in order to expand the repertoire of vertebrate species for muscle disease modelling, and to dissect further the functional interactions of dystrophin. We report here the identification of an apparent zebrafish orthologue of the human dystrophin gene that expresses a 400-kDa protein that is localised to the muscle membrane surface. These data suggest that the zebrafish may prove to be a beneficial vertebrate model to examine the role and functional interactions of dystrophin in disease and development.     

A Splice Site Mutation in Laminin-α2 Results in a Severe Muscular Dystrophy and Growth Abnormalities in Zebrafish

Congenital muscular dystrophy (CMD) is a clinically and genetically heterogeneous group of inherited muscle disorders. In patients, muscle weakness is usually present at or shortly after birth and is progressive in nature. Merosin deficient congenital muscular dystrophy (MDC1A) is a form of CMD caused by a defect in the laminin-α2 gene (LAMA2). Laminin-α2 is an extracellular matrix protein that interacts with the dystrophin-dystroglycan (DGC) complex in membranes providing stability to muscle fibers. In an N-ethyl-N-nitrosourea mutagenesis screen to develop zebrafish models of neuromuscular diseases, we identified a mutant fish that exhibits severe muscular dystrophy early in development. Genetic mapping identified a splice site mutation in the lama2 gene. This splice site is highly conserved in humans and this mutation results in mis-splicing of RNA and a loss of protein function. Homozygous lama2 mutant zebrafish, designated lama2(cl501/cl501), exhibited reduced motor function and progressive degeneration of skeletal muscles and died at 8-15 days post fertilization. The skeletal muscles exhibited damaged myosepta and detachment of myofibers in the affected fish. Laminin-α2 deficiency also resulted in growth defects in the brain and eye of the mutant fish. This laminin-α2 deficient mutant fish represents a novel disease model to develop therapies for modulating splicing defects in congenital muscular dystrophies and to restore the muscle function in human patients with CMD.     

Analysis of Zebrafish Larvae Skeletal Muscle Integrity with Evans Blue Dye

The zebrafish model is an emerging system for the study of neuromuscular disorders. In the study of neuromuscular diseases, the integrity of the muscle membrane is a critical disease determinant. To date, numerous neuromuscular conditions display degenerating muscle fibers with abnormal membrane integrity; this is most commonly observed in muscular dystrophies. Evans Blue Dye (EBD) is a vital, cell permeable dye that is rapidly taken into degenerating, damaged, or apoptotic cells; in contrast, it is not taken up by cells with an intact membrane. EBD injection is commonly employed to ascertain muscle integrity in mouse models of neuromuscular diseases. However, such EBD experiments require muscle dissection and/or sectioning prior to analysis. In contrast, EBD uptake in zebrafish is visualized in live, intact preparations. Here, we demonstrate a simple and straightforward methodology for performing EBD injections and analysis in live zebrafish. In addition, we demonstrate a co-injection strategy to increase efficacy of EBD analysis. Overall, this video article provides an outline to perform EBD injection and characterization in zebrafish models of neuromuscular disease.     

Destabilization of the dystrophin-glycoprotein complex without functional deficits in alpha-dystrobrevin null muscle

Alpha-dystrobrevin is a component of the dystrophin-glycoprotein complex (DGC) and is thought to have both structural and signaling roles in skeletal muscle. Mice deficient for alpha-dystrobrevin (adbn(-/-)) exhibit extensive myofiber degeneration and neuromuscular junction abnormalities. However, the biochemical stability of the DGC and the functional performance of adbn(-/-) muscle have not been characterized. Here we show that the biochemical association between dystrophin and beta-dystroglycan is compromised in adbn(-/-) skeletal muscle, suggesting that alpha-dystrobrevin plays a structural role in stabilizing the DGC. However, despite muscle cell death and DGC destabilization, costamere organization and physiological performance is normal in adbn(-/-) skeletal muscle. Our results demonstrate that myofiber degeneration alone does not cause functional deficits and suggests that more complex pathological factors contribute to the development of muscle weakness in muscular dystrophy.     

Microanatomy of adult zebrafish extraocular muscles

Binocular vision requires intricate control of eye movement to align overlapping visual fields for fusion in the visual cortex, and each eye is controlled by 6 extraocular muscles (EOMs). Disorders of EOMs are an important cause of symptomatic vision loss. Importantly, EOMs represent specialized skeletal muscles with distinct gene expression profile and susceptibility to neuromuscular disorders. We aim to investigate and describe the anatomy of adult zebrafish extraocular muscles (EOMs) to enable comparison with human EOM anatomy and facilitate the use of zebrafish as a model for EOM research. Using differential interference contrast (DIC), epifluorescence microscopy, and precise sectioning techniques, we evaluate the anatomy of zebrafish EOM origin, muscle course, and insertion on the eye. Immunofluorescence is used to identify components of tendons, basement membrane and neuromuscular junctions (NMJs), and to analyze myofiber characteristics. We find that adult zebrafish EOM insertions on the globe parallel the organization of human EOMs, including the close proximity of specific EOM insertions to one another. However, analysis of EOM origins reveals important differences between human and zebrafish, such as the common rostral origin of both oblique muscles and the caudal origin of the lateral rectus muscles. Thrombospondin 4 marks the EOM tendons in regions that are highly innervated, and laminin marks the basement membrane, enabling evaluation of myofiber size and distribution. The NMJs appear to include both en plaque and en grappe synapses, while NMJ density is much higher in EOMs than in somatic muscles. In conclusion, zebrafish and human EOM anatomy are generally homologous, supporting the use of zebrafish for studying EOM biology. However, anatomic differences exist, revealing divergent evolutionary pressures.     

Maturation and maintenance of the neuromuscular synapse: genetic evidence for roles of the dystrophin--glycoprotein complex

The dystrophin-glycoprotein complex (DGC) links the cytoskeleton of muscle fibers to their extracellular matrix. Using knockout mice, we show that a cytoplasmic DGC component, alpha-dystrobrevin (alpha-DB), is dispensable for formation of the neuromuscular junction (NMJ) but required for maturation of its postsynaptic apparatus. We also analyzed double and triple mutants lacking other cytoskeletal DGC components (utrophin and dystrophin) and myotubes lacking a alpha-DB or a transmembrane DGC component (dystroglycan). Our results suggest that alpha-DB acts via its linkage to the DGC to enhance the stability of postsynaptic specializations following their DGC-independent formation; dystroglycan may play additional roles in assembling synaptic basal lamina. Together, these results demonstrate involvement of distinct protein complexes in the formation and maintenance of the synapse and implicate the DGC in the latter process.     

The Roles of the Dystrophin-Associated Glycoprotein Complex at the Synapse

Duchenne muscular dystrophy is caused by mutations in the dystrophin gene and is characterized by progressive muscle wasting. A number of Duchenne patients also present with mental retardation. The dystrophin protein is part of the highly conserved dystrophin-associated glycoprotein complex (DGC) which accumulates at the neuromuscular junction (NMJ) and at a variety of synapses in the peripheral and central nervous systems. Many years of research into the roles of the DGC in muscle have revealed its structural function in stabilizing the sarcolemma. In addition, the DGC also acts as a scaffold for various signaling pathways. Here, we discuss recent advances in understanding DGC roles in the nervous system, gained from studies in both vertebrate and invertebrate model systems. From these studies, it has become clear that the DGC is important for the maturation of neurotransmitter receptor complexes and for the regulation of neurotransmitter release at the NMJ and central synapses. Furthermore, roles for the DGC have been established in consolidation of long-term spatial and recognition memory. The challenges ahead include the integration of the behavioral and mechanistic studies and the use of this information to identify therapeutic targets.     

Embryonic expression patterns of the Drosophila dystrophin-associated glycoprotein complex orthologs

Mutations in genes encoding proteins of the human dystrophin-associated glycoprotein complex (DGC) cause the Duchenne, Becker and limb-girdle muscular dystrophies. Subsets of the DGC proteins form tissue-specific complexes which are thought to play structural and signaling roles in the muscle and at the neuromuscular junction. Furthermore, mutations in the dystrophin gene that lead to Duchenne muscular dystrophy are frequently associated with cognitive and behavioral deficits, suggesting a role for dystrophin in the nervous system. Despite significant progress over the past decade, many fundamental questions about the roles played by dystrophin and the other DGC proteins in the muscle and peripheral and central nervous systems remain to be answered. Mammalian models of DGC gene function are complicated by the existence of fully or partially redundant genes whose functions can mask effects of the inactivation of a given DGC gene. The genome of the fruitfly Drosophila melanogaster encodes a single ortholog of the majority of the mammalian DGC protein subclasses, thus potentially simplifying their functional analysis. We report here the embryonic mRNA expression patterns of the individual DGC orthologs. We find that they are predominantly expressed in the nervous system and in muscle. Dystrophin, dystrobrevin-like, dystroglycan-like, syntrophin-like 1, and all three sarcoglycan orthologs are found in the brain and the ventral nerve cord, while dystrophin, dystrobrevin-like, dystroglycan-like, syntrophin-like 2, sarcoglycan alpha and sarcoglycan delta are expressed in distinct and sometimes overlapping domains of mesoderm-derived tissues, i.e. muscles of the body wall and around the gut.     

Expression of the dystrophin-glycoprotein complex is a marker for human airway smooth muscle phenotype maturation

Airway smooth muscle (ASM) cells may contribute to asthma pathogenesis through their capacity to switch between a synthetic/proliferative and a contractile phenotype. The multimeric dystrophin-glycoprotein complex (DGC) spans the sarcolemma, linking the actin cytoskeleton and extracellular matrix. The DGC is expressed in smooth muscle tissue, but its functional role is not fully established. We tested whether contractile phenotype maturation of human ASM is associated with accumulation of DGC proteins. We compared subconfluent, serum-fed cultures and confluent cultures subjected to serum deprivation, which express a contractile phenotype. Western blotting confirmed that beta-dystroglycan, beta-, delta-, and epsilon-sarcoglycan, and dystrophin abundance increased six- to eightfold in association with smooth muscle myosin heavy chain (smMHC) and calponin accumulation during 4-day serum deprivation. Immunocytochemistry showed that the accumulation of DGC subunits was specifically localized to a subset of cells that exhibit robust staining for smMHC. Laminin competing peptide (YIGSR, 1 microM) and phosphatidylinositol 3-kinase (PI3K) inhibitors (20 microM LY-294002 or 100 nM wortmannin) abrogated the accumulation of smMHC, calponin, and DGC proteins. These studies demonstrate that the accumulation of DGC is an integral feature for phenotype maturation of human ASM cells. This provides a strong rationale for future studies investigating the role of the DGC in ASM smooth muscle physiology in health and disease.     

An adhesion molecule involved in muscular dystrophies: structural and functional analysis of dystroglycan domains

Dystroglycan (DG) plays a pivotal role within the dystrophin-glycoprotein complex (DGC) which represents a major factor for muscle fibre stability upon contraction. It has been shown that many muscular dystrophy phenotypes are caused by mutations of proteins belonging to or being associated with the DGC. Due to its prominent role for muscle stability, the detailed knowledge of DG structural and functional aspects should be considered of primary importance in order to develop new treatments for neuromuscular diseases.     

Myotonia Congenita-Associated Mutations in Chloride Channel-1 Affect Zebrafish Body Wave Swimming Kinematics

Myotonia congenita is a human muscle disorder caused by mutations in CLCN1, which encodes human chloride channel 1 (CLCN1). Zebrafish is becoming an increasingly useful model for human diseases, including muscle disorders. In this study, we generated transgenic zebrafish expressing, under the control of a muscle specific promoter, human CLCN1 carrying mutations that have been identified in human patients suffering from myotonia congenita. We developed video analytic tools that are able to provide precise quantitative measurements of movement abnormalities in order to analyse the effect of these CLCN1 mutations on adult transgenic zebrafish swimming. Two new parameters for body-wave kinematics of swimming reveal changes in body curvature and tail offset in transgenic zebrafish expressing the disease-associated CLCN1 mutants, presumably due to their effect on muscle function. The capability of the developed video analytic tool to distinguish wild-type from transgenic zebrafish could provide a useful asset to screen for compounds that reverse the disease phenotype, and may be applicable to other movement disorders besides myotonia congenita.     

Paired Patch Clamp Recordings from Motor-neuron and Target Skeletal Muscle in Zebrafish

Larval zebrafish represent the first vertebrate model system to allow simultaneous patch clamp recording from a spinal motor-neuron and target muscle. This is a direct consequence of the accessibility to both cell types and ability to visually distinguish the single segmental CaP motor-neuron on the basis of morphology and location. This video demonstrates the microscopic methods used to identify a CaP motor-neuron and target muscle cells as well as the methodologies for recording from each cell type. Identification of the CaP motor-neuron type is confirmed by either dye filling or by the biophysical features such as action potential waveform and cell input resistance. Motor-neuron recordings routinely last for one hour permitting long-term recordings from multiple different target muscle cells. Control over the motor-neuron firing pattern enables measurements of the frequency-dependence of synaptic transmission at the neuromuscular junction. Owing to a large quantal size and the low noise provided by whole cell voltage clamp, all of the unitary events can be resolved in muscle. This feature permits study of basic synaptic properties such as release properties, vesicle recycling, as well as synaptic depression and facilitation. The advantages offered by this in vivo preparation eclipse previous neuromuscular model systems studied wherein the motor-neurons are usually stimulated by extracellular electrodes and the muscles are too large for whole cell patch clamp. The zebrafish preparation is amenable to combining electrophysiological analysis with a wide range of approaches including transgenic lines, morpholino knockdown, pharmacological intervention and in vivo imaging. These approaches, coupled with the growing number of neuromuscular disease models provided by mutant lines of zebrafish, open the door for new understanding of human neuromuscular disorders.     

FishNet: an online database of zebrafish anatomy

Background  

Over the last two decades, zebrafish have been established as a genetically versatile model system for investigating many different aspects of vertebrate developmental biology. With the credentials of zebrafish as a developmental model now well recognized, the emerging new opportunity is the wider application of zebrafish biology to aspects of human disease modelling. This rapidly increasing use of zebrafish as a model for human disease has necessarily generated interest in the anatomy of later developmental phases such as the larval, juvenile, and adult stages, during which many of the key aspects of organ morphogenesis and maturation take place. Anatomical resources and references that encompass these stages are non-existent in zebrafish and there is therefore an urgent need to understand how different organ systems and anatomical structures develop throughout the life of the fish.     

Muscular dystrophies involving the dystrophin-glycoprotein complex: an overview of current mouse models

The dystrophin-glycoprotein complex (DGC) is a multisubunit complex that connects the cytoskeleton of a muscle fiber to its surrounding extracellular matrix. Mutations in the DGC disrupt the complex and lead to muscular dystrophy. There are a few naturally occurring animal models of DGC-associated muscular dystrophy (e.g. the dystrophin-deficient mdx mouse, dystrophic golden retriever dog, HFMD cat and the delta-sarcoglycan-deficient BIO 14.6 cardiomyopathic hamster) that share common genetic protein abnormalities similar to those of the human disease. However, the naturally occurring animal models only partially resemble human disease. In addition, no naturally occurring mouse models associated with loss of other DGC components are available. This has encouraged the generation of genetically engineered mouse models for DGC-linked muscular dystrophy. Not only have analyses of these mice led to a significant improvement in our understanding of the pathogenetic mechanisms for the development of muscular dystrophy, but they will also be immensely valuable tools for the development of novel therapeutic approaches for these incapacitating diseases.     

Processing and assembly of the dystrophin glycoprotein complex

The assembly, processing and translocation of proteins occur constantly in all cells, and these processes also take place during the genesis, maintenance and repair of skeletal muscle. Skeletal muscle fibers are composed of myofibrils and are surrounded by a muscle plasma membrane, the sarcolemma. The sarcolemma serves as a docking location for many proteins. These proteins are important for establishing the physical connection between the extracellular matrix and the cytoskeleton and play a role in transmitting force related to muscle contraction. This physical connection is maintained through a myriad of proteins including the dystrophin glycoprotein complex (DGC). Normal sarcolemmal function requires proper DGC synthesis and positioning, and perturbation of the DGC leads to muscle membrane instability and disease.     

Membrane targeting and stabilization of sarcospan is mediated by the sarcoglycan subcomplex

The dystrophin-glycoprotein complex (DGC) is a multisubunit complex that spans the muscle plasma membrane and forms a link between the F-actin cytoskeleton and the extracellular matrix. The proteins of the DGC are structurally organized into distinct subcomplexes, and genetic mutations in many individual components are manifested as muscular dystrophy. We recently identified a unique tetraspan-like dystrophin-associated protein, which we have named sarcospan (SPN) for its multiple sarcolemma spanning domains (Crosbie, R.H., J. Heighway, D.P. Venzke, J.C. Lee, and K.P. Campbell. 1997. J. Biol. Chem. 272:31221-31224). To probe molecular associations of SPN within the DGC, we investigated SPN expression in normal muscle as a baseline for comparison to SPN's expression in animal models of muscular dystrophy. We show that, in addition to its sarcolemma localization, SPN is enriched at the myotendinous junction (MTJ) and neuromuscular junction (NMJ), where it is a component of both the dystrophin- and utrophin-glycoprotein complexes. We demonstrate that SPN is preferentially associated with the sarcoglycan (SG) subcomplex, and this interaction is critical for stable localization of SPN to the sarcolemma, NMJ, and MTJ. Our experiments indicate that assembly of the SG subcomplex is a prerequisite for targeting SPN to the sarcolemma. In addition, the SG- SPN subcomplex functions to stabilize alpha-dystroglycan to the muscle plasma membrane. Taken together, our data provide important information about assembly and function of the SG-SPN subcomplex.     

Muscle dystroglycan organizes the postsynapse and regulates presynaptic neurotransmitter release at the Drosophila neuromuscular junction

Background

The Dystrophin-glycoprotein complex (DGC) comprises dystrophin, dystroglycan, sarcoglycan, dystrobrevin and syntrophin subunits. In muscle fibers, it is thought to provide an essential mechanical link between the intracellular cytoskeleton and the extracellular matrix and to protect the sarcolemma during muscle contraction. Mutations affecting the DGC cause muscular dystrophies. Most members of the DGC are also concentrated at the neuromuscular junction (NMJ), where their deficiency is often associated with NMJ structural defects. Hence, synaptic dysfunction may also intervene in the pathology of dystrophic muscles. Dystroglycan is a central component of the DGC because it establishes a link between the extracellular matrix and Dystrophin. In this study, we focused on the synaptic role of Dystroglycan (Dg) in Drosophila.

Methodology/Principal Findings

We show that Dg was concentrated postsynaptically at the glutamatergic NMJ, where, like in vertebrates, it controls the concentration of synaptic Laminin and Dystrophin homologues. We also found that synaptic Dg controlled the amount of postsynaptic 4.1 protein Coracle and alpha-Spectrin, as well as the relative subunit composition of glutamate receptors. In addition, both Dystrophin and Coracle were required for normal Dg concentration at the synapse. In electrophysiological recordings, loss of postsynaptic Dg did not affect postsynaptic response, but, surprisingly, led to a decrease in glutamate release from the presynaptic site.

Conclusion/Significance

Altogether, our study illustrates a conservation of DGC composition and interactions between Drosophila and vertebrates at the synapse, highlights new proteins associated with this complex and suggests an unsuspected trans-synaptic function of Dg.     

Advanced models of human skeletal muscle differentiation,development and disease: Three-dimensional cultures,organoids and beyond

Advanced in vitro models of human skeletal muscle tissue are increasingly needed to model complex developmental dynamics and disease mechanisms not recapitulated in animal models or in conventional monolayer cell cultures. There has been impressive progress towards creating such models by using tissue engineering approaches to recapitulate a range of physical and biochemical components of native human skeletal muscle tissue. In this review, we discuss recent studies focussed on developing complex in vitro models of human skeletal muscle beyond monolayer cell cultures, involving skeletal myogenic differentiation from human primary myoblasts or pluripotent stem cells, often in the presence of structural scaffolding support. We conclude with our outlook on the future of advanced skeletal muscle three-dimensional cultures (e.g. organoids and biofabrication) to produce physiologically and clinically relevant platforms for disease modelling and therapy development in musculoskeletal and neuromuscular disorders.     

Two Dynamin-2 Genes Are Required for Normal Zebrafish Development

Dynamin-2 (DNM2) is a large GTPase involved in clathrin-mediated endocytosis and related trafficking pathways. Mutations in human DNM2 cause two distinct neuromuscular disorders: centronuclear myopathy and Charcot-Marie-Tooth disease. Zebrafish have been shown to be an excellent animal model for many neurologic disorders, and this system has the potential to inform our understanding of DNM2-related disease. Currently, little is known about the endogenous zebrafish orthologs to human DNM2. In this study, we characterize two zebrafish dynamin-2 genes, dnm2 and dnm2-like. Both orthologs are structurally similar to human DNM2 at the gene and protein levels. They are expressed throughout early development and in all adult tissues examined. Knockdown of dnm2 and dnm2-like gene products resulted in extensive morphological abnormalities during development, and expression of human DNM2 RNA rescued these phenotypes. Our findings suggest that dnm2 and dnm2-like are orthologs to human DNM2, and that they are required for normal zebrafish development.