Muscle could be the therapeutic target in SMA treatment

Efficient generation of a self-organizing neuromuscular junction model from human pluripotent stem cells

The complex neuromuscular network that controls body movements is the target of severe diseases that result in paralysis and death. Here, we report the development of a robust and efficient self-organizing neuromuscular junction (soNMJ) model from human pluripotent stem cells that can be maintained long-term in simple adherent conditions. The timely application of specific patterning signals instructs the simultaneous development and differentiation of position-specific brachial spinal neurons, skeletal muscles, and terminal Schwann cells. High-content imaging reveals self-organized bundles of aligned muscle fibers surrounded by innervating motor neurons that form functional neuromuscular junctions. Optogenetic activation and pharmacological interventions show that the spinal neurons actively instruct the synchronous skeletal muscle contraction. The generation of a soNMJ model from spinal muscular atrophy patient-specific iPSCs reveals that the number of NMJs and muscle contraction is severely affected, resembling the patient's pathology. In the future, the soNMJ model could be used for high-throughput studies in disease modeling and drug development. Thus, this model will allow us to address unmet needs in the neuromuscular disease field.

Muscle: an independent contributor to the neuromuscular spinal muscular atrophy disease phenotype

Spinal muscular atrophy (SMA) is a pediatric-onset neuromuscular disorder caused by insufficient survival motor neuron (SMN) protein. SMN restorative therapies are now approved for the treatment of SMA; however, they are not curative, likely due to a combination of imperfect treatment timing, inadequate SMN augmentation, and failure to optimally target relevant organs. Here, we consider the implications of imperfect treatment administration, focusing specifically on outcomes for skeletal muscle. We examine the evidence that muscle plays a contributing role in driving neuromuscular dysfunction in SMA. Next, we discuss how SMN might regulate the health of myofibers and their progenitors. Finally, we speculate on therapeutic outcomes of failing to raise muscle SMN to healthful levels and present strategies to restore function to this tissue to ensure better treatment results.

The Development of an Innovative Embedded Sensor for the Optical Measurement of Ex-Vivo Engineered Muscle Tissue Contractility

Tissue engineering is a multidisciplinary approach focused on the development of innovative bioartificial substitutes for damaged organs and tissues. For skeletal muscle, the measurement of contractile capability represents a crucial aspect for tissue replacement, drug screening and personalized medicine. To date, the measurement of engineered muscle tissues is rather invasive and not continuous. In this context, we proposed an innovative sensor for the continuous monitoring of engineered-muscle-tissue contractility through an embedded technique. The sensor is based on the calibrated deflection of one of the engineered tissue's supporting pins, whose movements are measured using a noninvasive optical method. The sensor was calibrated to return force values through the use of a step linear motor and a micro-force transducer. Experimental results showed that the embedded sensor did not alter the correct maturation of the engineered muscle tissue. Finally, as proof of concept, we demonstrated the ability of the sensor to capture alterations in the force contractility of the engineered muscle tissues subjected to serum deprivation.

The Importance of Biophysical and Biochemical Stimuli in Dynamic Skeletal Muscle Models

Classical approaches to engineer skeletal muscle tissue based on current regenerative and surgical procedures still do not meet the desired outcome for patient applications. Besides the evident need to create functional skeletal muscle tissue for the repair of volumetric muscle defects, there is also growing demand for platforms to study muscle-related diseases, such as muscular dystrophies or sarcopenia. Currently, numerous studies exist that have employed a variety of biomaterials, cell types and strategies for maturation of skeletal muscle tissue in 2D and 3D environments. However, researchers are just at the beginning of understanding the impact of different culture settings and their biochemical (growth factors and chemical changes) and biophysical cues (mechanical properties) on myogenesis. With this review we intend to emphasize the need for new in vitro skeletal muscle (disease) models to better recapitulate important structural and functional aspects of muscle development. We highlight the importance of choosing appropriate system components, e.g., cell and biomaterial type, structural and mechanical matrix properties or culture format, and how understanding their interplay will enable researchers to create optimized platforms to investigate myogenesis in healthy and diseased tissue. Thus, we aim to deliver guidelines for experimental designs to allow estimation of the potential influence of the selected skeletal muscle tissue engineering setup on the myogenic outcome prior to their implementation. Moreover, we offer a workflow to facilitate identifying and selecting different analytical tools to demonstrate the successful creation of functional skeletal muscle tissue. Ultimately, a refinement of existing strategies will lead to further progression in understanding important aspects of muscle diseases, muscle aging and muscle regeneration to improve quality of life of patients and enable the establishment of new treatment options.

A system for studying mechanisms of neuromuscular junction development and maintenance

The neuromuscular junction (NMJ), a cellular synapse between a motor neuron and a skeletal muscle fiber, enables the translation of chemical cues into physical activity. The development of this special structure has been subject to numerous investigations, but its complexity renders in vivo studies particularly difficult to perform. In vitro modeling of the neuromuscular junction represents a powerful tool to delineate fully the fine tuning of events that lead to subcellular specialization at the pre-synaptic and post-synaptic sites. Here, we describe a novel heterologous co-culture in vitro method using rat spinal cord explants with dorsal root ganglia and murine primary myoblasts to study neuromuscular junctions. This system allows the formation and long-term survival of highly differentiated myofibers, motor neurons, supporting glial cells and functional neuromuscular junctions with post-synaptic specialization. Therefore, fundamental aspects of NMJ formation and maintenance can be studied using the described system, which can be adapted to model multiple NMJ-associated disorders.

In vivo generation of a mature and functional artificial skeletal muscle

Extensive loss of skeletal muscle tissue results in mutilations and severe loss of function. In vitro-generated artificial muscles undergo necrosis when transplanted in vivo before host angiogenesis may provide oxygen for fibre survival. Here, we report a novel strategy based upon the use of mouse or human mesoangioblasts encapsulated inside PEG-fibrinogen hydrogel. Once engineered to express placental-derived growth factor, mesoangioblasts attract host vessels and nerves, contributing to in vivo survival and maturation of newly formed myofibres. When the graft was implanted underneath the skin on the surface of the tibialis anterior, mature and aligned myofibres formed within several weeks as a complete and functional extra muscle. Moreover, replacing the ablated tibialis anterior with PEG-fibrinogen-embedded mesoangioblasts also resulted in an artificial muscle very similar to a normal tibialis anterior. This strategy opens the possibility for patient-specific muscle creation for a large number of pathological conditions involving muscle tissue wasting.

Low levels of Survival Motor Neuron protein are sufficient for normal muscle function in the SMNΔ7 mouse model of SMA

Spinal Muscular Atrophy (SMA) is an autosomal recessive disorder characterized by loss of lower motor neurons. SMA is caused by deletion or mutation of the Survival Motor Neuron 1 (SMN1) gene and retention of the SMN2 gene. The loss of SMN1 results in reduced levels of the SMN protein. SMN levels appear to be particularly important in motor neurons; however SMN levels above that produced by two copies of SMN2 have been suggested to be important in muscle. Studying the spatial requirement of SMN is important in both understanding how SMN deficiency causes SMA and in the development of effective therapies. Using Myf5-Cre, a muscle-specific Cre driver, and the Cre-loxP recombination system, we deleted mouse Smn in the muscle of mice with SMN2 and SMNΔ7 transgenes in the background, thus providing low level of SMN in the muscle. As a reciprocal experiment, we restored normal levels of SMN in the muscle with low SMN levels in all other tissues. We observed that decreasing SMN in the muscle has no phenotypic effect. This was corroborated by muscle physiology studies with twitch force, tetanic and eccentric contraction all being normal. In addition, electrocardiogram and muscle fiber size distribution were also normal. Replacement of Smn in muscle did not rescue SMA mice. Thus the muscle does not appear to require high levels of SMN above what is produced by two copies of SMN2 (and SMNΔ7).

Skeletal muscle DNA damage precedes spinal motor neuron DNA damage in a mouse model of Spinal Muscular Atrophy (SMA)

Spinal Muscular Atrophy (SMA) is a hereditary childhood disease that causes paralysis by progressive degeneration of skeletal muscles and spinal motor neurons. SMA is associated with reduced levels of full-length Survival of Motor Neuron (SMN) protein, due to mutations in the Survival of Motor Neuron 1 gene. The mechanisms by which lack of SMN causes SMA pathology are not known, making it very difficult to develop effective therapies. We investigated whether DNA damage is a perinatal pathological event in SMA, and whether DNA damage and cell death first occur in skeletal muscle or spinal cord of SMA mice. We used a mouse model of severe SMA to ascertain the extent of cell death and DNA damage throughout the body of prenatal and newborn mice. SMA mice at birth (postnatal day 0) exhibited internucleosomal fragmentation in genomic DNA from hindlimb skeletal muscle, but not in genomic DNA from spinal cord. SMA mice at postnatal day 5, compared with littermate controls, exhibited increased apoptotic cell death profiles in skeletal muscle, by hematoxylin and eosin, terminal deoxynucleotidyl transferase dUTP nick end labeling, and electron microscopy. SMA mice had no increased cell death, no loss of choline acetyl transferase (ChAT)-positive motor neurons, and no overt pathology in the ventral horn of the spinal cord. At embryonic days 13 and 15.5, SMA mice did not exhibit statistically significant increases in cell death profiles in spinal cord or skeletal muscle. Motor neuron numbers in the ventral horn, as identified by ChAT immunoreactivity, were comparable in SMA mice and control littermates at embryonic day 15.5 and postnatal day 5. These observations demonstrate that in SMA, disease in skeletal muscle emerges before pathology in spinal cord, including loss of motor neurons. Overall, this work identifies DNA damage and cell death in skeletal muscle as therapeutic targets for SMA.

Neurogenic and myogenic contributions to hereditary motor neuron disease

Spinal muscular atrophy and spinal and bulbar muscular atrophy are characterized by lower motor neuron loss and muscle atrophy. Although it is accepted that motor neuron loss is a primary event in disease pathogenesis, inherent defects in muscle may also contribute to the disease progression and severity. In this review, we discuss the relative contributions of primary pathological processes in the motor axons, neuromuscular junctions and muscle to disease manifestations. Characterizing these contributions helps us to better understand the disease mechanisms and to better target therapeutic intervention.

Neuromuscular junction formation between human stem cell-derived motoneurons and human skeletal muscle in a defined system

Functional in vitro models composed of human cells will constitute an important platform in the next generation of system biology and drug discovery. This study reports a novel human-based in vitro Neuromuscular Junction (NMJ) system developed in a defined serum-free medium and on a patternable non-biological surface. The motoneurons and skeletal muscles were derived from fetal spinal stem cells and skeletal muscle stem cells. The motoneurons and skeletal myotubes were completely differentiated in the co-culture based on morphological analysis and electrophysiology. NMJ formation was demonstrated by phase contrast microscopy, immunocytochemistry and the observation of motoneuron-induced muscle contractions utilizing time-lapse recordings and their subsequent quenching by d-Tubocurarine. Generally, functional human based systems would eliminate the issue of species variability during the drug development process and its derivation from stem cells bypasses the restrictions inherent with utilization of primary human tissue. This defined human-based NMJ system is one of the first steps in creating functional in vitro systems and will play an important role in understanding NMJ development, in developing high information content drug screens and as test beds in preclinical studies for spinal or muscular diseases/injuries such as muscular dystrophy, Amyotrophic lateral sclerosis and spinal cord repair.

Engineering skeletal muscle tissue--new perspectives in vitro and in vivo

Muscle tissue engineering (TE) has not yet been clinically applied because of several problems. However, the field of skeletal muscle TE has been developing tremendously and new approaches and techniques have emerged. This review will highlight recent developments in the field of nanotechnology, especially electrospun nanofibre matrices, as well as potential cell sources for muscle TE. Important developments in cardiac muscle TE and clinical studies on Duchenne muscular dystrophy (DMD) will be included to show their implications on skeletal muscle TE.

Increased IGF-1 in muscle modulates the phenotype of severe SMA mice

Spinal muscular atrophy (SMA) is an inherited motor neuron disease caused by the mutation of the survival motor neuron 1 (SMN1) gene and deficiency of the SMN protein. Severe SMA mice have abnormal motor function and small, immature myofibers early in development suggesting that SMN protein deficiency results in retarded muscle growth. Insulin-like growth factor 1 (IGF-1) stimulates myoblast proliferation, induces myogenic differentiation and generates myocyte hypertrophy in vitro and in vivo. We hypothesized that increased expression of IGF-1 specifically in skeletal muscle would attenuate disease features of SMAΔ7 mice. SMAΔ7 mice overexpressing a local isoform of IGF-1 (mIGF-1) in muscle showed enlarged myofibers and a 40% increase in median survival compared with mIGF-1-negative SMA littermates (median survival = 14 versus 10 days, respectively, log-rank P = 0.025). Surprisingly, this was not associated with a significant improvement in motor behavior. Treatment of both mIGF-1(NEG) and mIGF-1(POS) SMA mice with the histone deacetylase inhibitor, trichostatin A (TSA), resulted in a further extension of survival and improved motor behavior, but the combination of mIGF-1 and TSA treatment was not synergistic. These results show that increased mIGF-1 expression restricted to muscle can modulate the phenotype of SMA mice indicating that therapeutics targeted to muscle alone should not be discounted as potential disease-modifying therapies in SMA. IGF-1 may warrant further investigation in mild SMA animal models and perhaps SMA patients.

Stem cell model of spinal muscular atrophy

Human embryonic stem cells provide a useful source of material for studying basic human development and various disease states. However, ethical issues concerning their procurement limit their acceptance and possible clinical applicability. Recent advances in stem cell technology have provided an alternative source of pluripotent stem cells that does not require the use of an embryo. This review addresses the generation of induced pluripotent stem cells from skin fibroblasts taken from various patient populations, with a specific focus on the pediatric disorder spinal muscular atrophy. These patient-derived cells may help researchers devise more appropriate therapies through a greater understanding of the molecular mechanisms that underlie neuron dysfunction and death in a variety of diseases. Furthermore, they provide an ideal platform for small-molecule screening and subsequent drug development.

Tissue engineering approach to repair abdominal wall defects using cell-seeded bovine tunica vaginalis in a rabbit model

The aim of this study was to engineer skeletal muscle tissue for repair abdominal wall defects. Myoblast were seeded onto the scaffolds and cultivated in vitro for 5 days. Full thickness abdominal wall defects (3 x 4 cm) were created in 18 male New Zealand white rabbits and randomly divided into two equal groups. The defects of the first group were repaired with myoblast-seeded-bovine tunica vaginalis whereas the second group repaired with non-seeded-bovine tunica vaginalis and function as a control. Three animals were sacrificed at 7th, 14th, and 30th days of post-implantation from each group and the explanted specimens were subjected to macroscopic and microscopic analysis. In every case, seeded scaffolds have better deposition of newly formed collagen with neo-vascularisation than control group. Interestingly, multinucleated myotubes and myofibers were only detected in cell-seeded group. This study demonstrated that myoblast-seeded-bovine tunica vaginalis can be used as an effective scaffold to repair severe and large abdominal wall defects with regeneration of skeletal muscle tissue.

Development and progress of engineering of skeletal muscle tissue

Engineering skeletal muscle tissue remains still a challenge, and numerous studies have indicated that this technique may be of great importance in medicine in the near future. This article reviews some of the recent findings resulting from tissue engineering science related to the contractile behavior and the phenotypes of muscle tissue cells in different three-dimensional environment, and discusses how tissue engineering could be used to create and regenerate skeletal muscle, as well as the extended applications and the related patents concerned with engineered skeletal muscle.

Focused in vivo genetic analysis of implanted engineered myofascial constructs

Successfully engineering functional muscle tissue either in vitro or in vivo to treat muscle defects rather than using the host muscle transfer would be revolutionary. Tissue engineering is on the cutting edge of biomedical research, bridging a gap between the clinic and the bench top. A new focus on skeletal muscle tissue engineering has led investigators to explore the application of satellite cells (autologous muscle precursor cells) as a vehicle for engineering tissues either in vitro or in vivo. However, few skeletal muscle tissue-engineering studies have reported on successful generation of living tissue substitutes for functional skeletal muscle replacement. Our model system combines a novel aligned collagen tube and autologous skeletal muscle satellite cells to create an engineered tissue repair for a surgically created ventral hernia as previously reported [SA Fann, L Terracio, W Yan, et al., A model of tissue-engineered ventral hernia repair, J Invest Surg. 2006;19(3):193-205]. Several key features we specifically observe are the significant persistence of transplanted skeletal muscle cell mass within the engineered repair, the integration of new tissue with adjacent native muscle, and the presence of significant neovascularization. In this study, we report on our experience investigating the genetic signals important to the integration of neoskeletal muscle tissue. The knowledge gained from our model system applies to the repair of severely injured extremities, maxillofacial reconstructions, and restorative procedures following tumor excision in other areas of the body.

Animal models of spinal muscular atrophy

Spinal muscular atrophy, a common autosomal recessive motor neuron disorder, is caused by the loss of the survival motor neuron gene (SMN1). SMN2, a nearly identical copy gene, is present in all spinal muscular atrophy patients but differs by a critical nucleotide that alters exon 7 splicing efficiency. This results in low survival motor neuron protein levels, which are not enough to sustain motor neurons. SMN disruption has been undertaken in different organisms (yeast, nematode, fly, zebrafish, and mouse) in an attempt to model this disease and gain fundamental knowledge about the survival motor neuron protein. This review compares the various animal models generated to date and summarizes a research picture that reveals a pleiotropic role for survival motor neuron protein; this summary also points to unique requirements for survival motor neuron protein in motor neurons. It is hoped that these observations will aid in pointing towards complementary paths for therapeutic discovery research.

Tissue engineering in head and neck reconstructive surgery: what type of tissue do we need?

Craniofacial tissue loss due to congenital defects, disease or injury is a major clinical problem. The head and neck region is composed of several tissues. The most prevalent method of reconstruction is autologous grafting. Often, there is insufficient host tissue for adequate repair of the defect side, and extensive donor site morbidity may result from the secondary surgical procedure. The field of tissue engineering has the potential to create functional replacements for damaged or pathologic tissues.

Cellular therapies in motor neuron diseases

Amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) are prototypical motor neuron diseases that result in progressive weakness as a result of motor neuron dysfunction and death. Though much work has been done in both diseases to identify the cellular mechanisms of motor neuron dysfunction, once motor neurons have died, one of potential therapies to restore function would be through the use of cellular transplantation. In this review, we discuss potential strategies whereby cellular therapies, including the use of stem cells, neural progenitors and cells engineered to secrete trophic factors, may be used in motor neuron diseases. We review pre-clinical data in rodents with each of these approaches and discuss advances and regulatory issues regarding the use of cellular therapies in human motor neuron diseases.

Spinal muscular atrophy: present state

Spinal muscular atrophy (SMA) is a hereditary neurodegenerative disease caused by homozygous deletions or mutations in the SMN1 gene on Chr.5q13. SMA spans from severe Werdnig-Hoffmann disease (SMA 1) to relatively benign Kugelberg-Welander disease (SMA 3). Onset before birth possibly aggravates the clinical course, because immature motoneurons do not show compensatory sprouting and collateral reinnervation, and motor units in SMA 1, in contrast to those in SMA 3, are not enlarged. Genetic evidence indicates that SMN2, a gene 99% identical to SMN1, can attenuate SMA severity: in patients, more SMN2 copies and higher SMN protein levels are correlated with milder SMA. There is evidence that SMN plays a role in motoneuron RNA metabolism, but it has also been linked to apoptosis. Several mouse models with motoneuron disease have been successfully treated with neurotrophic factors. None of these models is, however, homologous to SMA. Recently, genetic mouse models of SMA have been created by introducing human SMN2 transgenes into Smn knockout mice or by targeting the Smn gene knockout to neurons. These mice not only provide important insights into the pathogenesis of SMA but are also crucial for testing new therapeutic strategies. These include SMN gene transfer, molecules capable to up-regulate SMN expression and trophic or antiapoptotic factors.

Regular exercise prolongs survival in a type 2 spinal muscular atrophy model mouse

Several studies indicate that physical exercise is likely to be neuroprotective, even in the case of neuromuscular disease. In the present work, we evaluated the efficiency of running-based training on type 2 spinal muscular atrophy (SMA)-like mice. The model used in this study is an SMN (survival motor neuron)-null mouse carrying one copy of a transgene of human SMN2. The running-induced benefits sustained the motor function and the life span of the type 2 SMA-like mice by 57.3%. We showed that the extent of neuronal death is reduced in the lumbar anterior horn of the spinal cord of running-trained mice in comparison with untrained animals. Notably, exercise enhanced motoneuron survival. We showed that the running-mediated neuroprotection is related to a change of the alternative splicing pattern of exon 7 in the SMN2 gene, leading to increased amounts of exon 7-containing transcripts in the spinal cord of trained mice. In addition, analysis at the level of two muscles from the calf, the slow-twitch soleus and the fast-twitch plantaris, showed an overall conserved muscle phenotype in running-trained animals. These data provide the first evidence for the beneficial effect of exercise in SMA and might lead to important therapeutic developments for human SMA patients.

Spinal muscular atrophy: a deficiency in a ubiquitous protein; a motor neuron-specific disease

Spinal muscular atrophy (SMA) is a neurodegenerative disease in humans and the most common genetic cause of infant mortality. The disease results in motor neuron loss and skeletal muscle atrophy. Despite a range of disease phenotypes, SMA is caused by mutations in a single gene, the Survival of Motor Neuron 1 (SMN1) gene. Recent advances have shed light on functions of the protein product of this gene and the pathophysiology of the disease, yet, fundamental questions remain. This review attempts to highlight some of the recent advances made in the understanding of the disease and how loss of the ubiquitously expressed survival of motor neurons (SMN) protein results in the SMA phenotype. Answers to some of the questions raised may ultimately result in a viable treatment for SMA.

Hypomorphic Smn knockdown C2C12 myoblasts reveal intrinsic defects in myoblast fusion and myotube morphology

Dosage of the survival motor neuron (SMN) protein has been directly correlated with the severity of disease in patients diagnosed with spinal muscular atrophy (SMA). It is also clear that SMA is a neurodegenerative disorder characterized by the degeneration of the alpha-motor neurons in the anterior horn of the spinal cord and atrophy of the associated skeletal muscle. What is more controversial is whether it is neuronal and/or muscle-cell-autonomous defects that are responsible for the disease per se. Although motor neuron degeneration is generally accepted as the primary event in SMA, intrinsic muscle defects in this disease have not been ruled out. To gain a better understanding of the influence of SMN protein dosage in muscle, we have generated a hypomorphic series of myoblast (C2C12) stable cell lines with variable Smn knockdown. We show that depletion of Smn in these cells resulted in a decrease in the number of nuclear 'gems' (gemini of coiled bodies), reduced proliferation with no increase in cell death, defects in myoblast fusion, and malformed myotubes. Importantly, the severity of these abnormalities is directly correlated with the decrease in Smn dosage. Taken together, our work supports the view that there is an intrinsic defect in skeletal muscle cells of SMA patients and that this defect contributes to the overall pathogenesis in this devastating disease.

Skeletal muscle tissue engineering

The reconstruction of skeletal muscle tissue either lost by traumatic injury or tumor ablation or functional damage due to myopathies is hampered by the lack of availability of functional substitution of this native tissue. Until now, only few alternatives exist to provide functional restoration of damaged muscle tissues. Loss of muscle mass and their function can surgically managed in part using a variety of muscle transplantation or transposition techniques. These techniques represent a limited degree of success in attempts to restore the normal functioning, however they are not perfect solutions. A new alternative approach to addressing difficult tissue reconstruction is to engineer new tissues. Although those tissue engineering techniques attempting regeneration of human tissues and organs have recently entered into clinical practice, the engineering of skeletal muscle tissue ist still a scientific challenge. This article reviews some of the recent findings resulting from tissue engineering science related to the attempt of creation and regeneration of functional skeletal muscle tissue.

Y chromosome detection of three-dimensional tissue-engineered skeletal muscle constructs in a syngeneic rat animal model

Surgical reconstruction of muscle tissue lost by trauma or tumor ablation is limited by the lack of availability of functional native tissue substitution. Moreover, so far most inherited or acquired muscle diseases are lacking sufficient treatment, because only few alternatives exist to provide functional restoration of lost muscle tissues. Engineering those tissues and transplantation into sites of dysfunction may be an alternative approach and may allow replacement of such damaged or failing skeletal muscle tissues. Techniques attempting reconstruction of some human tissues and organs (tissue engineering) have been introduced into clinical practice recently. One major problem that previous transplantation studies were facing is the ability of detection of transplanted cells after integration. Using the Y chromosome in situ hybridization technique in a syngeneic rat model allows transplantation of cell constructs orthotopically, without manipulation of the cells, with no rejection or immunosuppression being implied, but providing a nondilutable genetic marker to identify transplanted cells. The purpose of our study was to create functional skeletal muscle tissue in vivo using the transplantation of primary myoblasts precultivated within a three-dimensional (3D) fibrin matrix and to determine the fate of the transplanted cells using the Y chromosome detection technique. 3D myoblast cultures were established derived from male donor rats and after 7 days of cultivation we performed an orthotopic transplantation of 3D cell constructs into a created muscle defect within the gracilis muscle of syngeneic female rats. Anti-desmin immunostaining and Y chromosome in situ hybridization indicated the survival and integration of transplanted male myoblasts into the female recipient animal, thus demonstrating the feasibility of this approach in tissue engineering and the research of cell transplantation in general.

Joint range of motion limitations in children and young adults with spinal muscular atrophy

OBJECTIVES

To elicit descriptive data about limited joint range of motion (ROM) in subjects with type II or III spinal muscular atrophy (SMA) and to examine the relation between the number of motions with limited range and both age and functional ability.

DESIGN

Descriptive cross-sectional study.

SETTING

Neurologic pediatric outpatient clinic at a hospital in Taiwan.

PARTICIPANTS

Twenty-seven subjects with SMA type II (mean age, 9.8+/-6.5y) and 17 with SMA type III (mean age, 12.2+/-8.7y). Intervention Measurement with transparent goniometers of joint ROM bilaterally of the shoulder, elbow, wrist, hip, knee, and ankle.

MAIN OUTCOME MEASURES

The proportion of participants with each ROM limitation compared with all participants with the same SMA type, age distribution of the participants with each ROM limitation, mean range loss of each motion limitation, and the contracture index (risk index of joint contracture).

RESULTS

Eighty-nine percent of the participants with SMA type II experienced knee extension limitation. Approximately 50% of the participants with both types of SMA had ankle dorsiflexion limitation. The motions of knee and hip extension and ankle dorsiflexion also had a relatively high contracture index. The number of motions with limited range positively correlated ( P <.001) with age and upper-extremity functional grade (the higher the functional grade, the poorer the functional ability) for SMA type II.

CONCLUSIONS

We found varying degrees of joint ROM limitation. Certain motions were noted to be high risks for the development of contractures. This risk was higher mostly in younger children.

Reduced expression of nicotinic AChRs in myotubes from spinal muscular atrophy I patients

Spinal muscular atrophy (SMA) is an autosomal recessive disorder characterized by degeneration of motoneurons and skeletal muscle atrophy. In its most severe form, it leads to death before the age of 2 years. While primary degeneration of motor neurons is well established in this disease, and this results in neurogenic atrophy of skeletal muscle, we have previously reported evidence for a primary muscle defect. In this study, we used primary cultures of embryonic human skeletal muscle cells from patients with SMA and from controls to examine the effects of muscle fiber differentiation in the absence of a nerve component. Cultured SMA skeletal muscle cells are unable to fuse correctly to form multinuclear myotubes, the precursors of the myofibers. We also show that agrin-induced aggregates of nicotinic acetylcholine receptors, one of the earliest steps of neuromuscular junction formation, cannot be visualized by confocal microscopy on cells from SMA patients. In binding experiments, we demonstrate that this lack of clustering is due to defective expression of the nicotinic acetylcholine receptors in the myotubes of SMA patients whereas the affinity of alpha-bungarotoxin for its receptor remains unchanged regardless of muscle cell type (SMA or control). These observations suggest that muscle cells from SMA patients have intrinsic abnormalities that may affect proper formation of the neuromuscular junction.

Engineering of muscle tissue

The loss or failure of an organ or tissue is one of the most frequent, devastating, and costly problems in health care. Tissue engineering and regenerative medicine is an emerging interdisciplinary field that applies the principles of biology and engineering to the development of viable substitutes that restore, maintain, or improve the function of human tissues and organs. Tissue engineering science has provided critical new knowledge that will deepen our understanding of the phenotype of an important category of cell types-the muscle cells-and this knowledge may enable meaningful advances in musculoskeletal tissue engineering. There are two principle strategies for the replacement of impaired muscle tissues. One approach uses the application of isolated and differentiated cells (in vivo tissue engineering), using a transport matrix for the cell delivery; the other uses in vitro-designed and pre-fabricated tissue equivalents (in vitro tissue engineering). Future developments and the decision regarding which approach is more promising depend on the elucidation of the relationships among cell growth and differentiation, the three-dimensional environment, the architecture of the cells, and gene expression of the developmental process and the survival of the cells and integration in the host in in vivo experiments. As the techniques of tissue engineering become more sophisticated and as issues such as vascularization and innervation are addressed, the usefulness of these methods for reconstructive surgery may grow significantly.

Possible pathogenic role of muscle cell dysfunction in motor neuron death in spinal muscular atrophy

We have previously shown that myofibers formed by fusion of muscle satellite cells from spinal muscular atrophy (SMA) I or II undergo degeneration 1 to 3 weeks after innervation by rat embryonic spinal cord explants, whereas normal myofibers survive for several months. In the "muscle component" of the coculture, the only cells responsible for the degeneration are the SMA muscle satellite cells. Moreover, SMA muscle satellite cells do not fuse as rapidly as do normal muscle satellite cells. To determine whether death of muscle cells precedes that of motor neurons, we studied the origin and kinetics of release of apoptotic microparticles. In SMA cocultures, motor neuron apoptosis occurred before myofiber degeneration becomes visible, indicating that SMA myofibers were unable to sustain survival of motor neurons. In normal cocultures, motor neuron apoptosis occurred 4 days after innervation. However, it did not continue beyond 2 days. These results strengthen the hypothesis that SMA is due to a defect in neurotrophic muscle cell function.

Construction of a plasmid containing human SMN, the SMA determining gene, coupled to EGFP

We describe here the construction of plasmid pEGFP-C3/SMN, bearing the human SMN gene coupled to the green fluorescent protein (GFP) sequence. The mutation of the SMN gene is responsible for spinal muscular atrophy (SMA), a frequent human infantile genetic disease. We introduced the SMN cDNA into the multiple cloning site of pEGFP-C3. This plasmid bears the neomycin-resistance sequence and the enhanced green fluorescent protein (EGFP). It results in the expression of a fusion protein bearing SMN coupled to a carboxy-terminal GFP tag, used for fluorescence localization studies. Transfection of primary human myoblasts with pEGFP-C3 or pEGFP-C3/SMN revealed that EGFP is intracellularly localized within the cytosol as well as in the nucleus, while the fusion protein EGFP-SMN localized within the nucleus in prominent dot-like structures termed "gems." These data demonstrate that human primary muscle cells can be efficiently transfected and may have important implications for the development of therapeutic strategies in SMA.

Normal innervation and differentiation of X-linked myotubular myopathy muscle cells in a nerve-muscle coculture system

To study the pathogenesis of X-linked recessive myotubular myopathy (XLMTM), we used a nerve-muscle coculture system which allows the reconstitution of functional motor units in vitro after coupling of human skeletal muscle cells with embryonic rat spinal cord explants. We used three skeletal muscle cell lines derived from subjects with known mutations in the MTM1 gene (two from embryonic tissues, associated with mutations predicted to give a severe phenotype, and one from a neonate still alive at 3 years 6 months and exhibiting a mild phenotype). We compared these three XLMTM muscle cell cultures with control cultures giving special attention to behaviour of living cocultures (formation of the myofibres, contractile activity, survival), expression of muscular markers (desmin, dystrophin, alpha-actinin, troponin-T, myosin heavy chain isoforms), and nerve-muscle interactions (expression and aggregation of the nicotinic acetylcholine receptors). We were unable to reproduce any 'myotubular' phenotype since XLMTM muscle cells behaved like normal cells with regard to all the investigated parameters. Our results suggest that XLMTM muscle might be intrinsically normal and emphasize the possible involvement of the myotubularin-deficient motor neurons in the development of the disease.

On the possible role of muscle in the pathogenesis of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a common human inherited disease characterized by degeneration of motoneurons and muscular atrophy. SMA results from deletions or mutations of the SMN (survival motor neuron) gene. A nerve-muscle coculture model, consisting of human muscle cells innervated by rat embryonic spinal cord explants, was used to study the pathogenesis of SMA. Previous studies have shown that myotubes formed by fusion of satellite muscle cells from patients with SMA I or SMA II (but not SMA III) underwent a characteristic degeneration 1-3 weeks after innervation. To correlate this cellular study with a molecular approach, we used reverse transcriptase-polymerase chain reaction (RT-PCR), and showed that SMN mRNAs were expressed throughout the fusion of normal satellite muscle cells with two peaks, the first appearing prior to the onset of fusion and the second one or two days before innervation. When satellite muscle cells from patients with SMA I or II were used, only the first peak was observed. Because in these cases the SMN telomeric gene (SMNtel) is deleted, it was concluded that the contribution of SMNtel-dependent mRNAs to the second peak is predominant in normal myogenesis and involved in maturation of myotubes. In addition, diseased satellite muscle cells did not fuse at the same rate as normal satellite muscle cells. Studies on myf-5, a muscle specific transcription factor family, showed that its expression was impaired during the fusion of satellite muscle cells from patients with SMA I or II compared with normal satellite muscle cells. Taken together, these observations suggest that (a) there is a muscle specific expression pattern of SMN, and (b) SMN probably plays a crucial role in maintenance of a functional motor unit, by allowing muscle cells to correctly differentiate and to allow motoneuron survival.

Cell types required to efficiently innervate human muscle cells in vitro

Previous studies carried out in our laboratory have shown that myofibers formed by fusion of muscle satellite cells from donors with spinal muscular atrophy (SMA) type I or II undergo a characteristic degeneration 1.5-3 weeks after innervation with rat embryonic spinal cord explants. The only cells responsible for degeneration of innervated cocultures are SMA muscle satellite cells. In order to study the kinetics of nerve and muscle cell degeneration in nerve-muscle cocultures implicating SMA muscle cells, we attempted to simplify the nervous component of the coculture and identify the nerve cell types necessary for a successful innervation. We demonstrate here that motoneurons alone were unable to innervate myotubes. However, when three cell types (motoneurons, sensory neurons, and Schwann cells) were added onto a reconstituted muscular component consisting of cloned muscle satellite cells and cloned muscular fibroblasts, myotubes contracted, indicating that functional neuromuscular junctions were formed. We concluded that the three cell types were required for a successful innervation. Moreover, we studied the effects of culture medium conditioned by different combinations of nerve cells on innervation; we observed that physical contacts among sensory neurons, motoneurons, and myotubes are required for a successful innervation; in contrast Schwann cells can be replaced by a Schwann-cell-conditioned medium, indicating that these cells produce a putative soluble "innervation-promoting factor." Obviously such a reconstituted system does not reflect the in vivo situation but it allows the formation of functional motor synapses and could therefore allow us to elucidate neuromuscular disease pathogenesis, especially that of spinal muscular atrophy.

Spinal muscular atrophy: molecular pathophysiology

Spinal muscular atrophy is an autosomal recessive disease characterized by motor neurone loss, muscle atrophy and weakness. Deletion or mutation of the SMN1 gene reduces intracellular survival motor neurone protein levels causes spinal muscular atrophy, most likely by interfering with spliceosome assembly. A range of clinical severity and corresponding survival motor neurone levels is seen because of the presence of copies of the transcriptionally inefficient SMN2 gene and possibly other modifying genes. The delineation of SMN1 as the gene that causes spinal muscular atrophy and the identification of genes that modify spinal muscular atrophy raise the prospect of gene therapy or in-vivo gene activation treatment for this frequently fatal disorder.