Another New “Super Muscle Stem Cell” Leaves Unaddressed the Real Problems of Cell Therapy for Duchenne Muscular Dystrophy

Identification and Characterization of Skeletal Muscle Stem Cells from Human Orbicularis Oculi Muscle

Skeletal muscle stem cell (SMSC) transplantation has shown great therapeutical potential in repairing muscle loss and dysfunction, but the muscle acquisition is usually a traumatic procedure causing pain and morbidity to the donor. In this study, we investigated the feasibility of isolating SMSCs from human orbicularis oculi muscle (OOM), which is routinely removed and discarded during ophthalmic cosmetic surgeries. OOM fragments were harvested from 18 female healthy donors undergoing upper eyelid plasties. Plastic-adherent cells were isolated from the muscles using a two-step plating method combined with collagenase digestion. A total of 15 cell cultures were successfully established from the muscle samples. These adherent cells were positive for the specific markers of SMSCs and could be directed toward the osteogenic, adipogenic, chondrogenic, and myogenic phenotypes in the presence of lineage-specific inductive media. Moreover, after cultured in the myogenic inductive medium for 3 weeks, the muscle cells were injected into the tibialis anterior muscles of nude mice and the cell fate was detected using a DiI-labeling technique. In vivo myogenesis was evidenced by the expression of DiI fluorescence after cell transplantation. The donor cells could be found in the satellite cell position and incorporated into the host myofibers. Our results demonstrated that human OOM represents a novel source of myogenic precursors with stem cell-like properties, which may provide a foundation for the SMSC-based therapeutics of skeletal muscle diseases.

Bioelectric Properties of Myogenic Progenitor Cells

Modern stem cell research has mainly focused on protein expression and transcriptional networks. However, transmembrane voltage gradients generated by ion channels and transporters have demonstrated to be powerful regulators of cellular processes. These physiological cues exert influence on cell behaviors ranging from differentiation and proliferation to migration and polarity. Bioelectric signaling is a fundamental element of living systems and an untapped reservoir for new discoveries. Dissecting these mechanisms will allow for novel methods of controlling cell fate and open up new opportunities in biomedicine. This review focuses on the role of ion channels and the resting membrane potential in the proliferation and differentiation of skeletal muscle progenitor cells. In addition, findings relevant to this topic are presented and potential implications for tissue engineering and regenerative medicine are discussed.

GNE Myopathy: Etiology, Diagnosis, and Therapeutic Challenges

GNE myopathy, previously known as hereditary inclusion body myopathy (HIBM), or Nonaka myopathy, is a rare autosomal recessive muscle disease characterized by progressive skeletal muscle atrophy. It has an estimated prevalence of 1 to 9:1,000,000. GNE myopathy is caused by mutations in the GNE gene which encodes the rate-limiting enzyme of sialic acid biosynthesis. The pathophysiology of the disease is not entirely understood, but hyposialylation of muscle glycans is thought to play an essential role. The typical presentation is bilateral foot drop caused by weakness of the anterior tibialis muscles with onset in early adulthood. The disease slowly progresses over the next decades to involve skeletal muscles throughout the body, with relative sparing of the quadriceps until late stages of the disease. The diagnosis of GNE myopathy should be considered in young adults presenting with bilateral foot drop. Histopathologic findings on muscle biopsies include fiber size variation, atrophic fibers, lack of inflammation, and the characteristic "rimmed" vacuoles on modified Gomori trichome staining. The diagnosis is confirmed by the presence of pathogenic (mostly missense) mutations in both alleles of the GNE gene. Although there is no approved therapy for this disease, preclinical and clinical studies of several potential therapies are underway, including substrate replacement and gene therapy-based strategies. However, developing therapies for GNE myopathy is complicated by several factors, including the rare incidence of disease, limited preclinical models, lack of reliable biomarkers, and slow disease progression.

Making muscle: skeletal myogenesis in vivo and in vitro

Skeletal muscle is the largest tissue in the body and loss of its function or its regenerative properties results in debilitating musculoskeletal disorders. Understanding the mechanisms that drive skeletal muscle formation will not only help to unravel the molecular basis of skeletal muscle diseases, but also provide a roadmap for recapitulating skeletal myogenesis in vitro from pluripotent stem cells (PSCs). PSCs have become an important tool for probing developmental questions, while differentiated cell types allow the development of novel therapeutic strategies. In this Review, we provide a comprehensive overview of skeletal myogenesis from the earliest premyogenic progenitor stage to terminally differentiated myofibers, and discuss how this knowledge has been applied to differentiate PSCs into muscle fibers and their progenitors in vitro.

An in vitro culture system that supports robust expansion and maintenance of in vivo engraftment capabilities for myogenic progenitor cells from adult mice

Muscle cell therapy and tissue engineering require large numbers of functional muscle precursor/progenitor cells (MPCs), making the in vitro expansion of MPCs a critical step for these applications. The cells must maintain their myogenic properties upon robust expansion, especially for cellular therapy applications, in order to achieve efficacious treatment. A major obstacle associated with MPCs expansion is the loss of "stemness," or regenerative capacity, of freshly isolated cells, presumably due to the absence of the native cellular niches. In the current study, we developed an in vitro system that allowed for long-term culture and massive expansion of murine MPCs (mMPCs) with the preservation of myogenic regeneration capabilities. Long term in vitro expanded mMPC expressed the myogenic stem cell markers Pax3 and Pax7 and formed spontaneously contracting myotubes. Furthermore, expanded mMPC injected into the tibialis anterior muscle of nude mice engrafted and formed myofibers. Collectively, the method developed in this study can be potentially adapted for the expansion of human MPCs to high enough numbers for treatment of muscle injuries in human patients.

Low Molecular Weight Dextran Sulfate Binds to Human Myoblasts and Improves their Survival after Transplantation in Mice

Myoblast transplantation represents a promising therapeutic strategy in the treatment of several genetic muscular disorders including Duchenne muscular dystrophy. Nevertheless, such an approach is impaired by the rapid death, limited migration, and rejection of transplanted myoblasts by the host. Low molecular weight dextran sulfate (DXS), a sulfated polysaccharide, has been reported to act as a cytoprotectant for various cell types. Therefore, we investigated whether DXS could act as a “myoblast protectant” either in vitro or in vivo after transplantation in immunodeficient mice. In vitro, DXS bound human myoblasts in a dose dependent manner and significantly inhibited staurosporine-mediated apoptosis and necrosis. DXS pretreatment also protected human myoblasts from natural killer cell-mediated cytotoxicity. When human myoblasts engineered to express the renilla luciferase transgene were transplanted in immunodeficient mice, bioluminescence imaging analysis revealed that the proportion of surviving myoblasts 1 and 3 days after transplantation was two times higher when cells were preincubated with DXS compared to control (77.9 ± 10.1% vs. 39.4 ± 4.9%, p = 0.0009 and 38.1 ± 8.5% vs. 15.1 ± 3.4%, p = 0.01, respectively). Taken together, we provide evidence that DXS acts as a myoblast protectant in vitro and is able in vivo to prevent the early death of transplanted myoblasts.

The skeletal muscle satellite cell: still young and fascinating at 50

The skeletal muscle satellite cell was first described and named based on its anatomic location between the myofiber plasma and basement membranes. In 1961, two independent studies by Alexander Mauro and Bernard Katz provided the first electron microscopic descriptions of satellite cells in frog and rat muscles. These cells were soon detected in other vertebrates and acquired candidacy as the source of myogenic cells needed for myofiber growth and repair throughout life. Cultures of isolated myofibers and, subsequently, transplantation of single myofibers demonstrated that satellite cells were myogenic progenitors. More recently, satellite cells were redefined as myogenic stem cells given their ability to self-renew in addition to producing differentiated progeny. Identification of distinctively expressed molecular markers, in particular Pax7, has facilitated detection of satellite cells using light microscopy. Notwithstanding the remarkable progress made since the discovery of satellite cells, researchers have looked for alternative cells with myogenic capacity that can potentially be used for whole body cell-based therapy of skeletal muscle. Yet, new studies show that inducible ablation of satellite cells in adult muscle impairs myofiber regeneration. Thus, on the 50th anniversary since its discovery, the satellite cell's indispensable role in muscle repair has been reaffirmed.

Three-dimensional porous scaffold allows long-term wild-type cell delivery in dystrophic muscle

Duchenne muscular dystrophy (DMD) is caused by the lack of dystrophin; affected muscles are characterized by continuous bouts of muscle degeneration, eventually leading to the exhaustion of the endogenous satellite cell pool. At present, only palliative treatments are available, although several gene and cell therapy-based approaches are being studied. In this study we proposed to overcome the limitations hampering intramuscular cell injection by using a biomaterial-based strategy. In particular, we used a three-dimensional (3D) collagen porous scaffold to deliver myogenic precursor cells (MPCs) in vivo in the mdx murine model of DMD. MPCs, derived from single fibres of wild-type donors, were expanded in vitro, seeded onto collagen scaffolds and implanted into the tibialis anterior muscles of normal and mdx mice. As a control, cells were delivered via direct intramuscular cell injection in the contralateral muscles. Scaffold-delivered MPCs displayed lower apoptosis and higher proliferation than injected cells; in terms of dystrophin restoration, collagen scaffolds yielded better results than direct injections. Importantly, time-course experiments indicated that the scaffolds acted as a cell reservoir, although cell migration was mostly contained within 400 µm from the scaffold-host tissue interface.

Emerging strategies for cell and gene therapy of the muscular dystrophies

The muscular dystrophies are a heterogeneous group of over 40 disorders that are characterised by muscle weakness and wasting. The most common are Duchenne muscular dystrophy and Becker muscular dystrophy, which result from mutations within the gene encoding dystrophin; myotonic dystrophy type 1, which results from an expanded trinucleotide repeat in the myotonic dystrophy protein kinase gene; and facioscapulohumeral dystrophy, which is associated with contractions in the subtelomeric region of human chromosome 1. Currently the only treatments involve clinical management of symptoms, although several promising experimental strategies are emerging. These include gene therapy using adeno-associated viral, lentiviral and adenoviral vectors and nonviral vectors, such as plasmid DNA. Exon-skipping and cell-based therapies have also shown promise in the effective treatment and regeneration of dystrophic muscle. The availability of numerous animal models for Duchenne muscular dystrophy has enabled extensive testing of a wide range of therapeutic approaches for this type of disorder. Consequently, we focus here on the therapeutic developments for Duchenne muscular dystrophy as a model of the types of approaches being considered for various types of dystrophy. We discuss the advantages and limitations of each therapeutic strategy, as well as prospects and recent successes in the context of future clinical applications.