Delta-sarcoglycan is necessary for early heart and muscle development in zebrafish

Modeling Sarcoglycanopathy in Danio rerio

Sarcoglycanopathies, also known as limb girdle muscular dystrophy 3-6, are rare muscular dystrophies characterized, although heterogeneous, by high disability, with patients often wheelchair-bound by late adolescence and frequently developing respiratory and cardiac problems. These diseases are currently incurable, emphasizing the importance of effective treatment strategies and the necessity of animal models for drug screening and therapeutic verification. Using the CRISPR/Cas9 genome editing technique, we generated and characterized δ-sarcoglycan and β-sarcoglycan knockout zebrafish lines, which presented a progressive disease phenotype that worsened from a mild larval stage to distinct myopathic features in adulthood. By subjecting the knockout larvae to a viscous swimming medium, we were able to anticipate disease onset. The δ-SG knockout line was further exploited to demonstrate that a δ-SG missense mutant is a substrate for endoplasmic reticulum-associated degradation (ERAD), indicating premature degradation due to protein folding defects. In conclusion, our study underscores the utility of zebrafish in modeling sarcoglycanopathies through either gene knockout or future knock-in techniques. These novel zebrafish lines will not only enhance our understanding of the disease's pathogenic mechanisms, but will also serve as powerful tools for phenotype-based drug screening, ultimately contributing to the development of a cure for sarcoglycanopathies.

Modeling Human Muscular Dystrophies in Zebrafish: Mutant Lines, Transgenic Fluorescent Biosensors, and Phenotyping Assays

Muscular dystrophies (MDs) are a heterogeneous group of myopathies characterized by progressive muscle weakness leading to death from heart or respiratory failure. MDs are caused by mutations in genes involved in both the development and organization of muscle fibers. Several animal models harboring mutations in MD-associated genes have been developed so far. Together with rodents, the zebrafish is one of the most popular animal models used to reproduce MDs because of the high level of sequence homology with the human genome and its genetic manipulability. This review describes the most important zebrafish mutant models of MD and the most advanced tools used to generate and characterize all these valuable transgenic lines. Zebrafish models of MDs have been generated by introducing mutations to muscle-specific genes with different genetic techniques, such as (i) N-ethyl-N-nitrosourea (ENU) treatment, (ii) the injection of specific morpholino, (iii) tol2-based transgenesis, (iv) TALEN, (v) and CRISPR/Cas9 technology. All these models are extensively used either to study muscle development and function or understand the pathogenetic mechanisms of MDs. Several tools have also been developed to characterize these zebrafish models by checking (i) motor behavior, (ii) muscle fiber structure, (iii) oxidative stress, and (iv) mitochondrial function and dynamics. Further, living biosensor models, based on the expression of fluorescent reporter proteins under the control of muscle-specific promoters or responsive elements, have been revealed to be powerful tools to follow molecular dynamics at the level of a single muscle fiber. Thus, zebrafish models of MDs can also be a powerful tool to search for new drugs or gene therapies able to block or slow down disease progression.

Development of a high-throughput tailored imaging method in zebrafish to understand and treat neuromuscular diseases

The zebrafish (Danio rerio) is a vertebrate species offering multitude of advantages for the study of conserved biological systems in human and has considerably enriched our knowledge in developmental biology and physiology. Being equally important in medical research, the zebrafish has become a critical tool in the fields of diagnosis, gene discovery, disease modeling, and pharmacology-based therapy. Studies on the zebrafish neuromuscular system allowed for deciphering key molecular pathways in this tissue, and established it as a model of choice to study numerous motor neurons, neuromuscular junctions, and muscle diseases. Starting with the similarities of the zebrafish neuromuscular system with the human system, we review disease models associated with the neuromuscular system to focus on current methodologies employed to study them and outline their caveats. In particular, we put in perspective the necessity to develop standardized and high-resolution methodologies that are necessary to deepen our understanding of not only fundamental signaling pathways in a healthy tissue but also the changes leading to disease phenotype outbreaks, and offer templates for high-content screening strategies. While the development of high-throughput methodologies is underway for motility assays, there is no automated approach to quantify the key molecular cues of the neuromuscular junction. Here, we provide a novel high-throughput imaging methodology in the zebrafish that is standardized, highly resolutive, quantitative, and fit for drug screening. By providing a proof of concept for its robustness in identifying novel molecular players and therapeutic drugs in giant axonal neuropathy (GAN) disease, we foresee that this new tool could be useful for both fundamental and biomedical research.

Regulation of gene expression during ontogeny of physiological function in the brackishwater amphipod Gammarus chevreuxi

Embryonic development is a complex process involving the co-ordinated onset and integration of multiple morphological features and physiological functions. While the molecular basis of morphological development in embryos is relatively well known for traditional model species, the molecular underpinning of the development of physiological functions is not. Here, we used global gene expression profiling to investigate the transcriptional changes associated with the development of morphological and physiological function in the amphipod crustacean Gammarus chevreuxi. We compared the transcriptomes at three timepoints during the latter half of development, characterised by different stages of the development of heart form and function: 10 days post fertilisation (dpf, Early: no heart structure visible), 15 dpf (Middle: heart present but not fully functional), and 18 dpf (Late: regular heartbeat). Gene expression profiles differed markedly between developmental stages, likely representing a change in the activity of different processes throughout the latter period of G. chevreuxi embryonic development. Differentially expressed genes belonged to one of three distinct clusters based on their expression patterns across development. One of these clusters, which included key genes relating to cardiac contractile machinery and calcium handling, displayed a pattern of sequential up-regulation throughout the developmental period studied. Further analyses of these transcripts could reveal genes that may influence the onset of a regular heartbeat. We also identified morphological and physiological processes that may occur alongside heart development, such as development of digestive caeca and the cuticle. Elucidating the mechanisms underpinning morphological and physiological development of non-model organisms will support improved understanding of conserved mechanisms, addressing the current phylogenetic gap between relatively well known model species.

Clinical and genetic spectrum of a large cohort of patients with δ-sarcoglycan muscular dystrophy

Sarcoglycanopathies include four subtypes of autosomal recessive limb-girdle muscular dystrophies (LGMDR3, LGMDR4, LGMDR5 and LGMDR6) that are caused, respectively, by mutations in the SGCA, SGCB, SGCG and SGCD genes. Delta-sarcoglycanopathy (LGMDR6) is the least frequent and is considered an ultra-rare disease. Our aim was to characterize the clinical and genetic spectrum of a large international cohort of LGMDR6 patients and to investigate whether or not genetic or protein expression data could predict diseasés severity. This is a retrospective study collecting demographic, genetic, clinical and histological data of patients with genetically confirmed LGMDR6 including protein expression data from muscle biopsies. We contacted 128 pediatric and adult neuromuscular units around the world that reviewed genetic data of patients with a clinical diagnosis of a neuromuscular disorder. We identified 30 patients with a confirmed diagnosis of LGMDR6 of which 23 patients were included in this study. Eighty seven percent of the patients had consanguineous parents. Ninety one percent of the patients were symptomatic at the time of the analysis. Proximal muscle weakness of the upper and lower limbs was the most common presenting symptom. Distal muscle weakness was observed early over the course of the disease in 56.5% of the patients. Cardiac involvement was reported in 5 patients (21.7%) and 4 patients (17.4%) required non-invasive ventilation. Sixty percent of patients were wheelchair-bound since early teens (median age of 12.0 years old). Patients with absent expression of the sarcoglycan complex on muscle biopsy had a significant earlier onset of symptoms and an earlier age of loss of ambulation compared to patients with residual protein expression. This study confirmed that delta-sarcoglycanopathy is an ultra-rare neuromuscular condition and described the clinical and molecular characteristics of the largest yet-reported collected cohort of patients. Our results showed that this is a very severe and quickly progressive disease characterized by generalized muscle weakness affecting predominantly proximal and distal muscles of the limbs. Similar to other forms of sarcoglycanopathies, the severity and rate of progressive weakness correlates inversely with the abundance of protein on muscle biopsy.

A novel inducible mutagenesis screen enables to isolate and clone both embryonic and adult zebrafish mutants

Conventional genetic screens for recessive mutants are inadequate for studying biological processes in the adult vertebrate due to embryonic lethality. Here, we report that a novel inducible mutagenesis system enables to study gene function in both embryonic and adult zebrafish. This system yields genetic mutants with conditional ectopic over- or under-expression of genes in F₁ heterozygotes by utilizing inducible Tet-On transcriptional activation of sense or anti-sense transcripts from entrapped genes by Tol2 transposase-meditated transgenesis. Pilot screens identified 37 phenotypic mutants displaying embryonic defects (34 lines), adult fin regeneration defects (7 lines), or defects at both stages (4 lines). Combination of various techniques (such as: generating a new mutant allele, injecting gene specific morpholino or mRNA etc) confirms that Dox-induced embryonic abnormalities in 10 mutants are due to dysfunction of entrapped genes; and that Dox-induced under-expression of 6 genes causes abnormal adult fin regeneration. Together, this work presents a powerful mutagenesis system for genetic analysis from zebrafish embryos to adults in particular and other model organisms in general.

Muscular dystrophy modeling in zebrafish

Skeletal muscle performs an essential function in human physiology with defects in genes encoding a variety of cellular components resulting in various types of inherited muscle disorders. Muscular dystrophies (MDs) are a severe and heterogeneous type of human muscle disease, manifested by progressive muscle wasting and degeneration. The disease pathogenesis and therapeutic options for MDs have been investigated for decades using rodent models, and considerable knowledge has been accumulated on the cause and pathogenetic mechanisms of this group of human disorders. However, due to some differences between disease severity and progression, what is learned in mammalian models does not always transfer to humans, prompting the desire for additional and alternative models. More recently, zebrafish have emerged as a novel and robust animal model for the study of human muscle disease. Zebrafish MD models possess a number of distinct advantages for modeling human muscle disorders, including the availability and ease of generating mutations in homologous disease-causing genes, the ability to image living muscle tissue in an intact animal, and the suitability of zebrafish larvae for large-scale chemical screens. In this chapter, we review the current understanding of molecular and cellular mechanisms involved in MDs, the process of myogenesis in zebrafish, and the structural and functional characteristics of zebrafish larval muscles. We further discuss the insights gained from the key zebrafish MD models that have been so far generated, and we summarize the attempts that have been made to screen for small molecules inhibitors of the dystrophic phenotypes using these models. Overall, these studies demonstrate that zebrafish is a useful in vivo system for modeling aspects of human skeletal muscle disorders. Studies using these models have contributed both to the understanding of the pathogenesis of muscle wasting disorders and demonstrated their utility as highly relevant models to implement therapeutic screening regimens.

Where do we stand in trial readiness for autosomal recessive limb girdle muscular dystrophies?

Autosomal recessive limb girdle muscular dystrophies (LGMD2) are a group of genetically heterogeneous diseases that are typically characterised by progressive weakness and wasting of the shoulder and pelvic girdle muscles. Many of the more than 20 different conditions show overlapping clinical features with other forms of muscular dystrophy, congenital, myofibrillar or even distal myopathies and also with acquired muscle diseases. Although individually extremely rare, all types of LGMD2 together form an important differential diagnostic group among neuromuscular diseases. Despite improved diagnostics and pathomechanistic insight, a curative therapy is currently lacking for any of these diseases. Medical care consists of the symptomatic treatment of complications, aiming to improve life expectancy and quality of life. Besides well characterised pre-clinical tools like animal models and cell culture assays, the determinants of successful drug development programmes for rare diseases include a good understanding of the phenotype and natural history of the disease, the existence of clinically relevant outcome measures, guidance on care standards, up to date patient registries, and, ideally, biomarkers that can help assess disease severity or drug response. Strong patient organisations driving research and successful partnerships between academia, advocacy, industry and regulatory authorities can also help accelerate the elaboration of clinical trials. All these determinants constitute aspects of translational research efforts and influence patient access to therapies. Here we review the current status of determinants of successful drug development programmes for LGMD2, and the challenges of translating promising therapeutic strategies into effective and accessible treatments for patients.

Influence of vascular endothelial growth factor stimulation and serum deprivation on gene activation patterns of human adipose tissue-derived stromal cells

Introduction

Stimulation of mesenchymal stromal cells and adipose tissue-derived stromal cells (ASCs) with vascular endothelial growth factor (VEGF) has been used in multiple animal studies and clinical trials for regenerative purposes. VEGF stimulation is believed to promote angiogenesis and VEGF stimulation is usually performed under serum deprivation. Potential regenerative molecular mechanisms are numerous and the role of contributing factors is uncertain. The aim of the current study was to investigate the effect of in vitro serum deprivation and VEGF stimulation on gene expression patterns of ASCs.

Methods

Gene expressions of ASCs cultured in complete medium, ASCs cultured in serum-deprived medium and ASCs stimulated with VEGF in serum-deprived medium were compared. ASC characteristics according to criteria set by the International Society of Cellular Therapy were confirmed by flow cytometry. Microarray gene expressions were obtained using the Affymetrix HT HG-U133+ GeneChip®. Gene set enrichment analysis was performed using the Kyoto Encyclopedia of Genes and Genomes and gene ontology terms. Transcription of selected genes of interest was confirmed by quantitative PCR.

Results

Compared to ASCs in complete medium, 190 and 108 genes were significantly altered by serum deprivation and serum deprivation combined with VEGF, respectively. No significant differences in gene expression patterns between serum-deprived ASCs and serum-deprived ASCs combined with VEGF stimulation were found. Genes most prominently and significantly upregulated by both conditions were growth factors (IGF1, BMP6, PDGFD, FGF9), adhesion molecule CLSTN2, extracellular matrix-related proteins such as matricellular proteins SMOC2, SPON1 and ADAMTS12, and inhibitors of proliferation (JAG1). The most significantly downregulated genes included matrix metalloproteinases (MMP3, MMP1), and proliferation markers (CDKN3) and GREM2 (a BMP6 antagonist).

Conclusion

The decisive factor for the observed change in ASC gene expression proves to be serum starvation rather than VEGF stimulation. Changes in expression of growth factors, matricellular proteins and matrix metalloproteinases in concert, diverge from direct pro-angiogenic paracrine mechanisms as a primary consequence of the used protocol. In vitro serum starvation (with or without VEGF present) appears to favour cardioprotection, extracellular matrix remodelling and blood vessel maturation relevant for the late maturation phase in infarct healing.

Spectrum of muscular dystrophies associated with sarcolemmal-protein genetic defects

Muscular dystrophies are heterogeneous genetic disorders that share progressive muscle wasting. This may generate partial impairment of motility as well as a dramatic and fatal course. Less than 30 years ago, the identification of the genetic basis of Duchenne muscular dystrophy opened a new era. An explosion of new information on the mechanisms of disease was witnessed, with many thousands of publications and the characterization of dozens of other genetic forms. Genes mutated in muscular dystrophies encode proteins of the plasma membrane and extracellular matrix, several of which are part of the dystrophin-associated complex. Other gene products localize at the sarcomere and Z band, or are nuclear membrane components. In the present review, we focus on muscular dystrophies caused by defects that affect the sarcolemmal and sub-sarcolemmal proteins. We summarize the nature of each disease, the genetic cause, and the pathogenic pathways that may suggest future treatment options. We examine X-linked Duchenne and Becker muscular dystrophies and the autosomal recessive limb-girdle muscular dystrophies caused by mutations in genes encoding sarcolemmal proteins. The mechanism of muscle damage is reviewed starting from disarray of the shock-absorbing dystrophin-associated complex at the sarcolemma and activation of inflammatory response up to the final stages of fibrosis. We trace only a part of the biochemical, physiopathological and clinical aspects of muscular dystrophy to avoid a lengthy list of different and conflicting observations. We attempt to provide a critical synthesis of what we consider important aspects to better understand the disease. In our opinion, it is becoming ever more important to go back to the bedside to validate and then translate each proposed mechanism. This article is part of a Special Issue entitled: Neuromuscular Diseases: Pathology and Molecular Pathogenesis.

KCTD10 is critical for heart and blood vessel development of zebrafish

KCTD10 is a member of the PDIP1 family, which is highly conserved during evolution, sharing a lot of similarities among human, mouse, and zebrafish. Recently, zebrafish KCTD13 has been identified to play an important role in the early development of brain and autism. However, the specific function of KCTD10 remains to be elucidated. In this study, experiments were carried out to determine the expression pattern of zebrafish KCTD10 mRNA during embryonic development. It was found that KCTD10 is a maternal gene and KCTD10 is of great importance in the shaping of heart and blood vessels. Our data provide direct clues that knockdown of KCTD10 resulted in severe pericardial edema and loss of heart formation indicated by morphological observation and crucial heart markers like amhc, vmhc, and cmlc2. The heart defect caused by KCTD10 is linked to RhoA and PCNA. Flk-1 staining revealed that intersomitic vessels were lost in the trunk, although angioblasts could migrate to the midline. These findings could be helpful to better understand the determinants responsible for the heart and blood vessel defects.

Zebrafish as a model of cardiac disease

The zebrafish has been rapidly adopted as a model for cardiac development and disease. The transparency of the embryo, its limited requirement for active oxygen delivery, and ease of use in genetic manipulations and chemical exposure have made it a powerful alternative to rodents. Novel technologies like TALEN/CRISPR-mediated genome engineering and advanced imaging methods will only accelerate its use. Here, we give an overview of heart development and function in the fish and highlight a number of areas where it is most actively contributing to the understanding of cardiac development and disease. We also review the current state of research on a feature that we only could wish to be conserved between fish and human; cardiac regeneration.

Swimming into prominence: the zebrafish as a valuable tool for studying human myopathies and muscular dystrophies

A new and exciting phase of muscle disease research has recently been entered. The application of next generation sequencing technology has spurred an unprecedented era of gene discovery for both myopathies and muscular dystrophies. Gene-based therapies for Duchenne muscular dystrophy have entered clinical trial, and several pathway-based therapies are doing so as well for a handful of muscle diseases. While many factors have aided the extraordinary developments in gene discovery and therapy development, the zebrafish model system has emerged as a vital tool in these advancements. In this review, we will highlight how the zebrafish has greatly aided in the identification of new muscle disease genes and in the recognition of novel therapeutic strategies. We will start with a general introduction to the zebrafish as a model, discuss the ways in which muscle disease can be modeled and analyzed in the fish, and conclude with observations from recent studies that highlight the power of the fish as a research tool for muscle disease.

Zebrafish models flex their muscles to shed light on muscular dystrophies

Muscular dystrophies are a group of genetic disorders that specifically affect skeletal muscle and are characterized by progressive muscle degeneration and weakening. To develop therapies and treatments for these diseases, a better understanding of the molecular basis of muscular dystrophies is required. Thus, identification of causative genes mutated in specific disorders and the study of relevant animal models are imperative. Zebrafish genetic models of human muscle disorders often closely resemble disease pathogenesis, and the optical clarity of zebrafish embryos and larvae enables visualization of dynamic molecular processes in vivo. As an adjunct tool, morpholino studies provide insight into the molecular function of genes and allow rapid assessment of candidate genes for human muscular dystrophies. This unique set of attributes makes the zebrafish model system particularly valuable for the study of muscle diseases. This review discusses how recent research using zebrafish has shed light on the pathological basis of muscular dystrophies, with particular focus on the muscle cell membrane and the linkage between the myofibre cytoskeleton and the extracellular matrix.

Neuromuscular disorders in zebrafish: state of the art and future perspectives

Neuromuscular disorders are a broad group of inherited conditions affecting the structure and function of the motor system with polymorphic clinical presentation and disease severity. Although individually rare, collectively neuromuscular diseases have an incidence of 1 in 3,000 and represent a significant cause of disability of the motor system. The past decade has witnessed the identification of a large number of human genes causing muscular disorders, yet the underlying pathogenetic mechanisms remain largely unclear, limiting the developing of targeted therapeutic strategies. To overcome this barrier, model systems that replicate the different steps of human disorders are increasingly being developed. Among these, the zebrafish (Danio rerio) has emerged as an excellent organism for studying genetic disorders of the central and peripheral motor systems. In this review, we will encounter most of the available zebrafish models for childhood neuromuscular disorders, providing a brief overview of results and the techniques, mainly transgenesis and chemical biology, used for genetic manipulation. The amount of data collected in the past few years will lead zebrafish to became a common functional tool for assessing rapidly drug efficacy and off-target effects in neuromuscular diseases and, furthermore, to shed light on new etiologies emerging from large-scale massive sequencing studies.

Orai1 deficiency leads to heart failure and skeletal myopathy in zebrafish

Mutations in the store-operated Ca²⁺ entry pore protein ORAI1 have been reported to cause myopathies in human patients but the mechanism involved is not known. Cardiomyocytes express ORAI1 but its role in heart function is also unknown. Using reverse genetics in zebrafish, we demonstrated that inactivation of the highly conserved zebrafish orthologue of ORAI1 resulted in severe heart failure, reduced ventricular systolic function, bradycardia and skeletal muscle weakness. Electron microscopy of Orai1-deficient myocytes revealed progressive skeletal muscle instability with loss of myofiber integrity and ultrastructural abnormalities of the z-disc in both skeletal and cardiac muscle. Isolated Orai1-deficient cardiomyocytes showed loss of the calcineurin-associated protein calsarcin from the z-discs. Furthermore, we found mechanosignal transduction was affected in Orai1-depleted hearts, indicating an essential role for ORAI1 in establishing the cardiac signaling transduction machinery at the z-disc. Our findings identify ORAI1 as an important regulator of cardiac and skeletal muscle function and provide evidence linking ORAI1-mediated calcium signaling to sarcomere integrity and cardiomyocyte function.

Filamin C plays an essential role in the maintenance of the structural integrity of cardiac and skeletal muscles, revealed by the medaka mutant zacro

Filamin C is an actin-crosslinking protein that is specifically expressed in cardiac and skeletal muscles. Although mutations in the filamin C gene cause human myopathy with cardiac involvement, the function of filamin C in vivo is not yet fully understood. Here we report a medaka mutant, zacro (zac), that displayed an enlarged heart, caused by rupture of the myocardiac wall, and progressive skeletal muscle degeneration in late embryonic stages. We identified zac to be a homozygous nonsense mutation in the filamin C (flnc) gene. The medaka filamin C protein was found to be localized at myotendinous junctions, sarcolemma, and Z-disks in skeletal muscle, and at intercalated disks in the heart. zac embryos showed prominent myofibrillar degeneration at myotendinous junctions, detachment of myofibrils from sarcolemma and intercalated disks, and focal Z-disk destruction. Importantly, the expression of γ-actin, which we observed to have a strong subcellular localization at myotendinous junctions, was specifically reduced in zac mutant myotomes. Inhibition of muscle contraction by anesthesia alleviated muscle degeneration in the zac mutant. These results suggest that filamin C plays an indispensable role in the maintenance of the structural integrity of cardiac and skeletal muscles for support against mechanical stress.

δ-Sarcoglycan-deficient muscular dystrophy: from discovery to therapeutic approaches

Mutations in the δ-sarcoglycan gene cause limb-girdle muscular dystrophy 2F (LGMD2F), an autosomal recessive disease that causes progressive weakness and wasting of the proximal limb muscles and often has cardiac involvement. Here we review the clinical implications of LGMD2F and discuss the current understanding of the putative mechanisms underlying its pathogenesis. Preclinical research has benefited enormously from various animal models of δ-sarcoglycan deficiency, which have helped researchers to explore therapeutic approaches for both muscular dystrophy and cardiomyopathy.

Lack of Apobec2-related proteins causes a dystrophic muscle phenotype in zebrafish embryos

Apo2 proteins interact with chaperone Unc45b (but not Hsp90) and are required for correct zebrafish skeletal musculature and heart function.

The chaperones Unc45b and Hsp90a are essential for folding of myosin in organisms ranging from worms to humans. We show here that zebrafish Unc45b, but not Hsp90a, binds to the putative cytidine deaminase Apobec2 (Apo2) in an interaction that requires the Unc45/Cro1p/She4p-related (UCS) and central domains of Unc45b. Morpholino oligonucleotide-mediated knockdown of the two related proteins Apo2a and Apo2b causes a dystrophic phenotype in the zebrafish skeletal musculature and impairs heart function. These phenotypic traits are shared with mutants of unc45b, but not with hsp90a mutants. Apo2a and -2b act nonredundantly and bind to each other in vitro, which suggests a heteromeric functional complex. Our results demonstrate that Unc45b and Apo2 proteins act in a Hsp90a-independent pathway that is required for integrity of the myosepta and myofiber attachment. Because the only known function of Unc45b is that of a chaperone, Apo2 proteins may be clients of Unc45b but other yet unidentified processes cannot be excluded.

Sarcoglycanopathies: molecular pathogenesis and therapeutic prospects

Sarcoglycanopathies are a group of autosomal recessive muscle-wasting disorders caused by genetic defects in one of four cell membrane glycoproteins, alpha-, beta-, gamma- or delta-sarcoglycan. These four sarcoglycans form a subcomplex that is closely linked to the major dystrophin-associated protein complex, which is essential for membrane integrity during muscle contraction and provides a scaffold for important signalling molecules. Proper assembly, trafficking and targeting of the sarcoglycan complex is of vital importance, and mutations that severely perturb tetramer formation and localisation result in sarcoglycanopathy. Gene defects in one sarcoglycan cause the absence or reduced concentration of the other subunits. Most genetic defects generate mutated proteins that are degraded through the cell's quality control system; however, in many cases, conformational modifications do not affect the function of the protein, yet it is recognised as misfolded and prematurely degraded. Recent evidence shows that misfolded sarcoglycans could be rescued to the cell membrane by assisting their maturation along the ER secretory pathway. This review summarises the etiopathogenesis of sarcoglycanopathies and highlights the quality control machinery as a potential pharmacological target for therapy of these genetic disorders.

Biphasic myopathic phenotype of mouse DUX, an ORF within conserved FSHD-related repeats

Facioscapulohumeral muscular dystrophy (FSHD) is caused by contractions of D4Z4 repeats at 4q35.2 thought to induce misregulation of nearby genes, one of which, DUX4, is actually localized within each repeat. A conserved ORF (mDUX), embedded within D4Z4-like repeats, encoding a double-homeodomain protein, was recently identified on mouse chromosome 10. We show here that high level mDUX expression induces myoblast death, while low non-toxic levels block myogenic differentiation by down-regulating MyoD and Myf5. Toxicity and MyoD/Myf5 expression changes were competitively reversed by overexpression of Pax3 or Pax7, implying mechanistic similarities with the anti-myogenic activity of human DUX4. We tested the effect of mDUX expression on Xenopus development, and found that global overexpression led to abnormalities in gastrulation. When targeted unilaterally into blastomeres fated to become tail muscle in 16-cell embryos, mDUX caused markedly reduced tail myogenesis on the injected side. These novel cell and animal models highlight the myopathic nature of sequences within the FSHD-related repeat array.

Myogenesis and molecules - insights from zebrafish Danio rerio

Myogenesis is a fundamental process governing the formation of muscle in multicellular organisms. Recent studies in zebrafish Danio rerio have described the molecular events occurring during embryonic morphogenesis and have thus greatly clarified this process, helping to distinguish between the events that give rise to fast v. slow muscle. Coupled with the well-known Hedgehog signalling cascade and a wide variety of cellular processes during early development, the continual research on D. rerio slow muscle precursors has provided novel insights into their cellular behaviours in this organism. Similarly, analyses on fast muscle precursors have provided knowledge of the behaviour of a sub-set of epitheloid cells residing in the anterior domain of somites. Additionally, the findings by various groups on the roles of several molecules in somitic myogenesis have been clarified in the past year. In this study, the authors briefly review the current trends in the field of research of D. rerio trunk myogenesis.

Muscle degeneration and leukocyte infiltration caused by mutation of zebrafish Fad24

Factor for adipocyte differentiation 24 (fad24) is a novel gene that has been implicated in adipocyte differentiation and DNA replication. In a screen for zebrafish mutants that have an abnormal tissue distribution of neutrophils, we identified an insertional allele of fad24, fad24hi1019. Homozygous fad24hi1019 larvae exhibit muscle degeneration accompanied by leukocyte infiltration. Muscle degeneration was extensive and included tissue apoptosis and disorganized, poorly striated muscle fibers. Blocking apoptosis using pan-caspase inhibitors resulted in decreased neutrophil recruitment into the body of the larva, suggesting a causative link between apoptosis and leukocyte infiltration. These findings suggest that zebrafish is a powerful genetic model system to address the interplay between muscle degeneration and leukocyte infiltration, and indicate that tissue apoptosis may contribute to neutrophil recruitment in some inflammatory states.

Genetic isolation and characterization of a splicing mutant of zebrafish dystrophin

Sapje-like (sap(cl100)) was one of eight potential zebrafish muscle mutants isolated as part of an early-pressure screen of 500 families. This mutant shows a muscle tearing phenotype similar to sapje (dys-/-) and both mutants fail to genetically complement suggesting they have a mutation in the same gene. Protein analysis confirms a lack of dystrophin in developing sapje-like embryos. Sequence analysis of the sapje-like dystrophin mRNA shows that exon 62 is missing in the dystrophin transcript causing exon 63 to be translated out of frame terminating translation at a premature stop codon at the end of exon 63. Sequence analysis of sapje-like genomic DNA identified a mutation in the donor splice junction at the end of dystrophin exon 62. This mutation is similar to splicing mutations associated with human forms of Duchenne Muscular Dystrophy. Sapje-like is the first zebrafish dystrophin splicing mutant identified to date and represents a novel disease model which can be used in future studies to identify therapeutic compounds for treating diseases caused by splicing defects.

Specific knockdown of delta-sarcoglycan gene in C2C12 in vitro causes post-translational loss of other sarcoglycans without mechanical stress

The precise role of delta-sarcoglycan (SG) that is constitutively expressed in skeletal muscle cells and may serve for maintaining the sarcolemmal integrity has not been identified. The delta-SG protein is at first among SG complex. To specifically identify the role in C(2)C(12) cells during the myogenesis, we screened several RNA interference (RNAi) candidates at first, and knocked down both levels of the mRNA and protein, employing adenovirus-mediated RNAi. We found no morphological alteration at both myoblast and myotube stages by suppression of delta-SG. The specific knockdown of delta-SG accompanied a concomitant decrease of alpha-, beta-, and gamma-SGs preserving normal levels of each transcript. As for the localization, alpha-, beta-, and gamma-SGs were weakly stained on the cell membrane in delta-SG knockdown cells, whereas each SG in control cell was localized both on the cell membrane and myoplasm abundantly. This enhanced post-translational loss would represent similitude of the progression of cardiomuscular diseases in vitro. Different from cardiac muscle cells, skeletal muscle cell culture without muscle contraction may imply that mechanical stress per se is not primarily involved in the progression of limb-girdle muscular dystrophy. Furthermore, we have observed translocation of calpain-2 to cell membrane in delta-SG knockdown cells, suggesting that Ca(2+)-sensitive proteases, calpains closely take part in post-translational proteolysis.

Therapeutic possibilities in the autosomal recessive limb-girdle muscular dystrophies

Fourteen years ago, the first disease-causing mutation in a form of autosomal recessive limb-girdle muscular dystrophy was reported. Since then the number of genes has been extended to at least 14 and the phenotypic spectrum has been broadened. The generation of mouse models helped to improve our understanding of the pathogenesis of the disease and also served to study therapeutic possibilities. All autosomal recessive limb-girdle muscular dystrophies are rare diseases, which is one reason why there have been so very few controlled clinical trials. Other reasons are insufficient natural history data and the lack of standardized assessment criteria and validated outcome measures. Currently, therapeutic possibilities are mainly restricted to symptomatic treatment and the treatment of disease complications. On the other hand, new efforts in translational research and the development of molecular therapeutic approaches suggest that more promising clinical trials will be carried out in autosomal recessive limb-girdle muscular dystrophy in the next several years.

Cascade effect of cardiac myogenesis gene expression during cardiac looping in tbx5 knockdown zebrafish embryos

Zebrafish tbx5 expresses in the heart, pectoral fins and eyes of zebrafish during embryonic development. In zebrafish, injection of tbx5 morpholino antisense RNA caused changes of heart conformation, defect of heart looping, pericardium effusion, dropsy of ventral position and decreased heart rate. We suggested that cardiac myogenesis genes might be responsible for this phenomenon. Morpholino antisense RNA which against the initiation site of tbx5 gene was designed in order to knockdown the expression of tbx5, and the results were analyzed by whole-mount in situ hybridization and quantitative real-time PCR. Expression of cardiac myogenesis genes amhc, vmhc and cmlc2 were expressed constantly at the early embryonic development and reached its highest rate right before cardiac looping initiated. These cardiac myogenesis genes showed insufficient expressions within different heart defect embryos. Moreover, vmhc showed ectopic expression in addition to heart looping defect in heart defective embryos at 36 hpf. Our data suggests that the heart failure caused by the knockdown of tbx5 gene might result from the down-regulation of cardiac myogenesis genes.

Modeling human muscle disease in zebrafish

Zebrafish reproduce in large quantities, grow rapidly, and are transparent early in development. For these reasons, zebrafish have been used extensively to model vertebrate development and disease. Like mammals, zebrafish express dystrophin and many of its associated proteins early in development and these proteins have been shown to be vital for zebrafish muscle stability. In dystrophin-null zebrafish, muscle degeneration becomes apparent as early as 3 days post-fertilization (dpf) making the zebrafish an excellent organism for large-scale screens to identify other genes involved in the disease process or drugs capable of correcting the disease phenotype. Being transparent, developing zebrafish are also an ideal experimental model for monitoring the fate of labeled transplanted cells. Although zebrafish dystrophy models are not meant to replace existing mammalian models of disease, experiments requiring large numbers of animals may be best performed in zebrafish. Results garnered from using this model could lead to a better understanding of the pathogenesis of the muscular dystrophies and the development of future therapies.

Dynamic changes in the subcellular localization of Drosophila beta-sarcoglycan during the cell cycle

One of the proposed roles of sarcoglycan is to stabilize dystrophin glycoprotein complexes in muscle sarcolemma. Involvement in signal transduction has also been proposed and abnormalities in some sarcoglycan genes are known to be responsible for muscular dystrophy. While characterization of sarcoglycans in muscle has been performed, little is known about its functions in the non-muscle tissues in which mammalian sarcoglycans are expressed. Here, we investigated temporal and spatial expression patterns of Drosophila beta-sarcoglycan (dScgbeta) during development by immunohistochemistry. In addition to almost ubiquitous expression in various tissues and organs, as seen for its mammalian counterpart, anti-dScgbeta staining data of embryos, eye imaginal discs, and salivary glands demonstrated cytoplasmic localization during S phase in addition to plasma membrane staining. Furthermore we found that subcellular localization of dScgbeta dramatically changes during mitosis through possible association with tubulin. These observations point to a complex role of sarcoglycans in non-muscle tissues.