Development of Approaches to Improve Cell Survival in Myoblast Transfer Therapy

Myoblast transplantation has been extensively studied as a gene complementation approach for genetic diseases such as Duchenne Muscular Dystrophy. This approach has been found capable of delivering dystrophin, the product missing in Duchenne Muscular Dystrophy muscle, and leading to an increase of strength in the dystrophic muscle. This approach, however, has been hindered by numerous limitations, including immunological problems, and low spread and poor survival of the injected myoblasts. We have investigated whether antiinflammatory treatment and use of different populations of skeletal muscle–derived cells may circumvent the poor survival of the injected myoblasts after implantation. We have observed that different populations of muscle-derived cells can be isolated from skeletal muscle based on their desmin immunoreactivity and differentiation capacity. Moreover, these cells acted differently when injected into muscle: 95% of the injected cells in some populations died within 48 h, while others richer in desmin-positive cells survived entirely. Since pure myoblasts obtained from isolated myofibers and myoblast cell lines also displayed a poor survival rate of the injected cells, we have concluded that the differential survival of the populations of muscle-derived cells is not only attributable to their content in desmin-positive cells. We have observed that the origin of the myogenic cells may influence their survival in the injected muscle. Finally, we have observed that myoblasts genetically engineered to express an inhibitor of the inflammatory cytokine, IL-1, can improve the survival rate of the injected myoblasts. Our results suggest that selection of specific muscle-derived cell populations or the control of inflammation can be used as an approach to improve cell survival after both myoblast transplantation and the myoblast-mediated ex vivo gene transfer approach.     

alpha7 integrin expressing human fetal myogenic progenitors have stem cell-like properties and are capable of osteogenic differentiation

During muscle development, precursor cells fuse to form myofibers. Following injury in adult muscle, quiescent satellite cells become activated to regenerate muscle in a fashion similar to fetal development. Recent studies indicate that murine skeletal myoblasts can differentiate along multiple cell lineages including the osteoblastic pathway. However, little is known about the multipotency of human myogenic cells. Here, we isolate myogenic precursor cells from human fetal and adult muscle by sorting for the laminin-binding alpha7 integrin and demonstrate their differentiation potential and alteration in adhesive behavior. The alpha7-positive human fetal progenitors were efficient at forming myotubes and a majority expressed known muscle markers including M-cadherin and c-Met, but were heterogeneous for desmin and MyoD expression. To test their pluripotent differentiation potential, enriched populations of alpha7-positive fetal cells were subjected to inductive protocols. Although the myoblasts appeared committed to a muscle lineage, they could be converted to differentiate along the osteoblastic pathway in the presence of BMP-2. Interestingly, osteogenic cells showed altered adhesion and migratory activity that reflected growth factor-induced changes in integrin expression. These results indicate that alpha7-expressing fetal myoblasts are capable of differentiation to osteoblast lineage with a coordinated switch in integrin profiles and may represent a mechanism that promotes homing and recruitment of myogenic stem cells for tissue repair and remodeling.     

Inhibition of miR-214 expression represses proliferation and differentiation of C2C12 myoblasts

MicroRNAs (miRNAs) are small non-coding RNAs that participate in diverse biological processes including skeletal muscle development. MiR-214 is an miRNA that is differentially expressed in porcine embryonic muscle and adult skeletal muscle, suggesting that miR-214 may be related to embryonic myogenesis. In this study, the myoblast cell line C2C12 was used for functional analysis of miR-214 in vitro. The results showed that miR-214 was expressed both in myoblasts and in myotubes and was upregulated during differentiation. After treatment with an miR-214 inhibitor and culturing in differentiation medium, myoblast differentiation was repressed, as indicated by the significant downregulation of expression of the myogenic markers myogenin and myosin heavy chain (MyHC). Interestingly, myoblast proliferation was also repressed when cells were transfected with an miR-214 inhibitor and cultured in growth medium by real-time proliferation assay and cell cycle analysis. Our results showed that miR-214 regulates both proliferation and differentiation of myoblasts depending on the conditions.     

Generation of skeletal muscle cells from embryonic and induced pluripotent stem cells as an in vitro model and for therapy of muscular dystrophies

Muscular dystrophies (MDs) are a heterogeneous group of inherited disorders characterized by progressive muscle wasting and weakness likely associated with exhaustion of muscle regeneration potential. At present, no cures or efficacious treatments are available for these diseases, but cell transplantation could be a potential therapeutic strategy. Transplantation of myoblasts using satellite cells or other myogenic cell populations has been attempted to promote muscle regeneration, based on the hypothesis that the donor cells repopulate the muscle and contribute to its regeneration. Embryonic stem cells (ESCs) and more recently induced pluripotent stem cells (iPSCs) could generate an unlimited source of differentiated cell types, including myogenic cells. Here we review the literature regarding the generation of myogenic cells considering the main techniques employed to date to elicit efficient differentiation of human and murine ESCs or iPSCs into skeletal muscle. We also critically analyse the possibility of using these cellular populations as an alternative source of myogenic cells for cell therapy of MDs.     

Skeletal muscle satellite cells appear during late chicken embryogenesis.

The emergence of avian satellite cells during development has been studied using markers that distinguish adult from fetal cells. Previous studies by us have shown that myogenic cultures from fetal (Embryonic Day 10) and adult 12-16 weeks) chicken pectoralis muscle (PM) each regulate expression of the embryonic isoform of fast myosin heavy chain (MHC) differently. In fetal cultures, embryonic MHC is coexpressed with a ventricular MHC in both myocytes (differentiated myoblasts) and myotubes. In contrast, myocytes and newly formed myotubes in adult cultures express ventricular but not embryonic MHC. In the current study, the appearance of myocytes and myotubes which express ventricular but not embryonic MHC was used to determine when adult myoblasts first emerge during avian development. By examining patterns of MHC expression in mass and clonal cultures prepared from embryonic and posthatch chicken skeletal muscle using double-label immunofluorescence with isoform-specific monoclonal antibodies, we show that a significant number of myocytes and myotubes which stain for ventricular but not embryonic MHC are first seen in cultures derived from PM during fetal development (Embryonic Day 18) and comprise the majority, if not all, of the myoblasts present at hatching and beyond. These results suggest that adult type myoblasts become dominant in late embryogenesis. We also show that satellite cell cultures derived from adult slow muscle give results similar to those of cultures derived from adult fast muscle. Cultures derived from Embryonic Day 10 hindlimb form myocytes and myotubes that coexpress ventricular and embryonic MHCs in a manner similar to cells of the Embryonic Day 10 PM. Thus, adult and fetal expression patterns of ventricular and embryonic MHCs are correlated with developmental age but not muscle fiber type.     

Coexpression of α-sarcomeric actin, α-smooth muscle actin and desmin during myogenesis in rat and mouse embryos I. Skeletal muscle

Expression of vimentin, desmin, alpha-sarcomeric and alpha-smooth muscle actins in embryonic tissues of rat and mice was examined using an immunohistochemical approach. The results showed a similarity in the expression of desmin and alpha-actin isoforms (alpha-sr and alpha-sm) in skeletal muscle cells during murine feto-embryonic development. In the two species, coexpression of alpha-sr and alpha-sm actins has been observed in cardiomyoblasts, myotomal myoblasts and myotubes. The intensity of alpha-sm actin expression decreased during the terminal steps of myogenesis and disappeared completely in mature cardiomyocytes and myofibres. Desmin was expressed in all prefusion myoblasts (type 1 and 2 myoblasts), myotubes, and in myofibres. The appearance of desmin in myoblasts of somites preceded by a few hours the expression of the alpha-actins (alpha-sr and alpha-sm). Our study on vimentin expression, limited to rat embryos, revealed that somite premyoblasts expressed only vimentin, type 1 myoblasts expressed vimentin and desmin, and type 2 myoblasts (rhabdomyoblasts) expressed desmin and alpha-actins (alpha-sr and alpha-sm). Our study demonstrates the resemblance between feto-embryonic myogenesis and myogenic neoplastic differentiation: desmin appears before the alpha-actins in embryonic myoblasts, and can be considered as a marker of an initial step in myogenic differentiation. alpha-sm actin, considered as a striated muscle cell feto-embryonic actin, is expressed transiently in skeletal myoblasts and cardiomyoblasts during development and reappears during neoplastic transformation of skeletal muscle.     

MyoD and myogenin expression patterns in cultures of fetal and adult chicken myoblasts.

Isolated chicken myoblasts had previously been utilized in many studies aiming at understanding the emergence and regulation of the adult myogenic precursors (satellite cells). However, in recent years only a small number of chicken satellite cell studies have been published compared to the increasing number of studies with rodent satellite cells. In large part this is due to the lack of markers for tracing avian myogenic cells before they become terminally differentiated and express muscle-specific structural proteins. We previously demonstrated that myoblasts isolated from fetal and adult chicken muscle display distinct schedules of myosin heavy-chain isoform expression in culture. We further showed that myoblasts isolated from newly hatched and young chickens already possess the adult myoblast phenotype. In this article, we report on the use of polyclonal antibodies against the chicken myogenic regulatory factor proteins MyoD and myogenin for monitoring fetal and adult chicken myoblasts as they progress from proliferation to differentiation in culture. Fetal-type myoblasts were isolated from 11-day-old embryos and adult-type myoblasts were isolated from 3-week-old chickens. We conclude that fetal myoblasts express both MyoD and myogenin within the first day in culture and rapidly transit into the differentiated myosin-expressing state. In contrast, adult myoblasts are essentially negative for MyoD and myogenin by culture Day 1 and subsequently express first MyoD and then myogenin before expressing sarcomeric myosin. The delayed MyoD-to-myogenin transition in adult myoblasts is accompanied by a lag in the fusion into myotubes, compared to fetal myoblasts. We also report on the use of a commercial antibody against the myocyte enhancer factor 2A (MEF2A) to detect terminally differentiated chicken myoblasts by their MEF2+ nuclei. Collectively, the results support the hypothesis that fetal and adult myoblasts represent different phenotypic populations. The fetal myoblasts may already be destined for terminal differentiation at the time of their isolation, and the adult myoblasts may represent progenitors that reside in an earlier compartment of the myogenic lineage.     

CARP,a Myostatin-downregulated Gene in CFM Cells,Is a Novel Essential Positive Regulator of Myogenesis

Myostatin, a member of the TGF-β superfamily, has been shown to act as a negative regulator of myogenesis. Although its role in myogenesis has been clearly documented through genetic analysis, few gene cascades that respond to myostatin signaling and regulate myogenesis have been characterized, especially in avian species. In a previous study, we screened for such genes in chicken fetal myoblasts (CFMs) using the differential display PCR method and found that cardiac ankyrin repeat protein (CARP) was downregulated by myostatin and specifically expressed in chicken skeletal muscle. However, little is known about the potential functions of CARP in chicken skeletal myogenesis. In this study, the expression patterns of chicken CARP and the possible function of this gene in skeletal muscle growth were characterized. Our data showed that CARP was predominantly expressed in postnatal skeletal muscle, and its expression increased during myogenic differentiation in CFM cells. When CARP was overexpressed, CFM cell growth was enhanced by accelerating the cell cycle at the G1 to S phase transition and increasing cyclin D1 expression. CARP knockdown had the opposite effect: while myoblasts underwent differentiation, knockdown of CARP expression induced extensive cell death, suppressed the formation of myotubes, and markedly decreased the expression of differentiation-related genes such as myosin heavy chain (MHC), myoD, and caveolin-3. Our findings indicate that CARP may play a key role in the myostatin signaling cascade that governs chicken skeletal myogenesis through promoting proliferation and avoiding apoptosis during CFM cell differentiation.     

Exosomes derived from inflammatory myoblasts promote M1 polarization and break the balance of myoblast proliferation/differentiation

BACKGROUNDAcute muscle injuries are one of the most common injuries in sports. Severely injured muscles are prone to re-injury due to fibrotic scar formation caused by prolonged inflammation. How to regulate inflammation and suppress fibrosis is the focus of promoting muscle healing. Recent studies have found that myoblasts and macrophages play important roles in the inflammatory phase following muscle injury; however, the crosstalk between these two types of cells in the inflammatory environment, particularly the exosome-related mechanisms, had not been well studied. AIMTo evaluate the effects of exosomes from inflammatory C2C12 myoblasts (IF-C2C12-Exos) on macrophage polarization and myoblast proliferation/differentiation.METHODSA model of inflammation was established in vitro by lipopolysaccharide stimulation of myoblasts. C2C12-Exos were isolated and purified from the supernatant of myoblasts by gradient centrifugation. Multiple methods were used to identify the exosomes. Gradient concentrations of IF-C2C12-Exos were added to normal macrophages and myoblasts. PKH67 fluorescence tracing was used to identify the interaction between exosomes and cells. Microscopic morphology, Giemsa stain, and immunofluorescence were carried out for histological analysis. Additionally, ELISA assays, flow cytometry, and western blot were conducted to analyze molecular changes. Moreover, myogenic proliferation was assessed by the BrdU test, scratch assay, and CCK-8 assay.RESULTSWe found that the PKH-67-marked C2C12-Exos can be endocytosed by both macrophages and myoblasts. IF-C2C12-Exos induced M1 macrophage polarization and suppressed the M2 phenotype in vitro. In addition, these exosomes also stimulated the inflammatory reactions of macrophages. Furthermore, we demonstrated that IF-C2C12-Exos disrupted the balance of myoblast proliferation/differentiation, leading to enhanced proliferation and suppressed fibrogenic/myogenic differentiation.CONCLUSIONIF-C2C12-Exos can induce M1 polarization, resulting in a sustained and aggravated inflammatory environment that impairs myoblast differentiation, and leads to enhanced myogenic proliferation. These results demonstrate why prolonged inflammation occurs after acute muscle injury and provide a new target for the regulation of muscle regeneration.     

Satellite cells, myoblasts and other occasional myogenic progenitors: possible origin, phenotypic features and role in muscle regeneration

In the vertebrate embryo, skeletal muscle originates from somites and is formed in discrete steps by different classes of progenitor cells. After myotome formation, embryonic myoblasts give rise to primary fibers in the embryo, while fetal myoblasts give rise to secondary fibers, initially smaller and surrounding primary fibers. Satellite cells appear underneath the newly formed basal lamina that develops around each fiber, and contribute to post-natal growth and regeneration of muscle fibers. Recently, different types of non somitic stem-progenitor cells have been shown to contribute to muscle regeneration. The origin of these different cell types and their possible lineage relationships with other myogenic cells as well as their possible role in muscle regeneration will be discussed. Finally, possible use of different myogenic cells in experimental protocols of cell therapy will be briefly outlined.     

An alternate protocol for establishment of primary caprine fetal myoblast cell culture: an in vitro model for muscle growth study

Cultured myoblasts have been used extensively as an in vitro model in understanding the underlying mechanisms of myogenesis. Various protocols for establishing a pure myoblast culture have been reported which involve the use of special procedures like flow cytometry and density gradient centrifugation. In goat, only a few protocols for establishing a myogenic cell culture are available and these protocols use adult muscle tissues which often does not yield sufficient numbers of precursor cells with adequate proliferative capacity. Considering the disadvantages of adult myoblasts, we are proposing an alternate protocol using caprine fetus which does not require any special procedures. In the present study, more than 90–95% fetal-derived cell populations had the typical spindle to polyhedral shape of myoblast cell and stained positive for desmin, hence confirming their myogenic origin. These cells attained the maximum confluency as early as 3–4 d against 3 wk by adult myoblasts indicating a better growth potential. Further, quantitative real-time PCR analysis revealed a higher expression (p?<?0.01) of myogenic regulatory factors (i.e., myogenic determination factor 1, myogenic factor 5, and myogenin) and myostatin (MSTN) in the fetal as compared to the adult myoblasts. Consequently, higher proliferation and differentiation ability along with higher abundance of myogenic markers and MSTN make the fetal myoblasts a better in vitro model.     

干扰Sema7A抑制C2C12成肌细胞的增殖和分化

为研究脑信号蛋白家族（Semaphorins）成员Sema7A对成肌细胞增殖和分化的影响,本文设计并合成了Sema7A基因的小干扰RNA（small interfering RNA,siRNA）,用此siRNA转染C2C12成肌细胞．通过Hoechst核染和流式细胞术检测细胞增殖情况,免疫荧光检测肌管的形成情况,real-time qPCR和Western印迹技术检测成肌标记基因的变化.结果显示,干扰Sema7A后,C2C12成肌细胞增殖减慢,处在G2和S期的细胞所占的比例明显下降,而G1期细胞的比例升高．免疫荧光检测结果显示,干扰Sema7A后,肌管的直径及MyHC+细胞所占比例均显著降低．Real-time qPCR和Western印迹结果也显示,肌肉分化标志基因MyoD、MyoG、MyHC的mRNA及蛋白质表达均下降．进一步检测Sema7A受体下游信号通路发现,干扰Sema7A后,其下游信号分子PI3K和AKT的磷酸化水平被下调．以上结果表明,Sema7A可以调节C2C12成肌细胞的增殖和分化,可能是通过其受体作用于PI3K/AKT信号通路实现的,这为进一步研究Sema7A在骨骼肌发育中的作用提供实验基础.     

Distinct myogenic programs of embryonic and fetal mouse muscle cells: expression of the perinatal myosin heavy chain isoform in vitro.

Early embryonic and late fetal mouse myogenic cells showed distinct patterns of perinatal myosin heavy chain (MHC) isoform expression upon differentiation in vitro. In cultures of somite or limb muscle cells isolated from Day 9 to Day 12 embryos, differentiated cells that expressed perinatal MHC were rare and perinatal MHC was not detectable by immunoblotting. In cultures of limb muscle cells isolated from Day 13 to Day 18 fetuses, in contrast, the perinatal MHC isoform was easily detected and was expressed in a substantial percentage of myocytes and myotubes. Analyses of clonally derived muscle colonies and cytosine arabinoside-treated fetal muscle cell cultures suggested that different fetal muscle cell nuclei initiated perinatal MHC expression at different times. In both embryonic and fetal cell cultures, the embryonic MHC isoform was expressed by all differentiated cells examined. A small number of myotubes in fetal muscle cell cultures showed a mosaic distribution of MHC isoform accumulation in which the perinatal MHC isoform accumulated in a restricted region of the myotube near particular nuclei, whereas the embryonic MHC isoform accumulated throughout the myotube. Thus, the myogenic program of fetal, but not embryonic, mouse myogenic cells includes expression of the perinatal MHC isoform upon differentiation in culture.     

Geometric control of myogenic cell fate

This work combines expertise in stem cell biology and bioengineering to define the system for geometric control of proliferation and differentiation of myogenic progenitor cells. We have created an artificial niche of myogenic progenitor cells, namely, modified extracellular matrix (ECM) substrates with spatially embedded growth or differentiation factors (GF, DF) that predictably direct muscle cell fate in a geometric pattern. Embedded GF and DF signal progenitor cells from specifically defined areas on the ECM successfully competed against culture media for myogenic cell fate determination at a clearly defined boundary. Differentiation of myoblasts into myotubes is induced in growth-promoting medium, myotube formation is delayed in differentiation-promoting medium, and myogenic cells, at different stages of proliferation and differentiation, can be induced to coexist adjacently in identical culture media. This method can be used to identify molecular interactions between cells in different stages of myogenic differentiation, which are likely to be important determinants of tissue repair. The designed ECM niches can be further developed into a vehicle for transplantation of myogenic progenitor cells maintaining their regenerative potential. Additionally, this work may also serve as a general model to engineer synthetic cellular niches to harness the regenerative potential of organ stem cells.