Myogenic cell lines derived from transgenic mice carrying a thermolabile T antigen: a model system for the derivation of tissue-specific and mutation-specific cell lines

A dual-marker system for quantitative studies of myoblast transplantation in the mouse

BACKGROUND

Myoblast transplantation (MT) is a potential approach for gene transfer into skeletal muscle, the efficiency of which depends upon the number of copies of donor genome incorporated into the host tissue. We have developed a system for quantitative studies of MT that measures amounts of donor-derived genome in host muscles and estimates the contributions of donor cell survival and proliferation in vivo.

METHODS

[14C]thymidine-labeled, male myoblasts were transplanted into female muscles, providing two donor cell markers, Y chromosome and [14C]. The markers were measured in muscle extracts by slot blotting and scintillation counting, respectively.

RESULTS

In each extract, the amount of Y chromosome was used to quantify donor-derived genome, whereas the radiolabel provided an estimate of cell survival. Furthermore, the different modes of inheritance of the markers meant that proliferation of surviving donor cells was detected as a change in marker ratio.

CONCLUSIONS

This system provides a method for assessing potential improvements of MT.

Increase in the proliferative capacity of human myoblasts by using the T antigen under the vimentin promoter control

Normal myoblasts have a strictly limited growth potential and senesce after a defined number of population doubling. The objective of this study was to determine whether the proliferative capacity of human myoblasts could be extended without inhibiting myogenic differentiation. We have established a stable transfected human myoblast cell line that expresses the SV 40 large T antigen under the control of the human vimentin promoter. We show that these cells have an increased proliferative capacity compared with that of normal myoblasts. Indeed, the final proliferative capacity was increased to 19 passages (5 for normal myoblasts). Moreover, they retained their capacity to differentiate fully, as indicated by their morphology and electrophysiological properties as well as by the expression of different markers of differentiation. The generation of human myogenic cell lines with the ability to proliferate for a longer period of time than primary myoblasts and while retaining the capacity to differentiate into myotubes could provide a valuable tool for the derivation of cell lines from human diseased muscle cells.

Viral gene delivery to skeletal muscle: insights on maturation-dependent loss of fiber infectivity for adenovirus and herpes simplex type 1 viral vectors

The mechanisms causing age-dependent loss of muscle fiber infectivity observed in vivo for both adenoviral (Ad) and herpes simplex virus type 1 (HSV-1) gene delivery vectors remain poorly understood. Here we investigate the possible bases for this phenomenon using the novel application of enzymatically isolated, viable, single muscle fibers. We show that maturation-dependent loss of fiber infectivity is recapitulated in single fibers, and, thus, is not solely due to host immune response. Using localized irradiation of muscle in vivo, we show data suggesting that Ad infectivity of differentiated myofibers depends, at least in part, on myoblasts to mediate fiber transduction. On the other hand, infection of single fibers by HSV-1 is not affected by irradiation. Using confocal microscopy, we show that the basal lamina of myogenic cells efficiently infected by HSV-1 is structurally less organized than that of fibers resistant to infection by HSV-1. As well, we show that single myofibers isolated from adult, basal lamina-defective mice (merosin-deficient, dy/dy) are at least 10-fold more susceptible to infection by HSV-1 than are myofibers isolated from control mice. Together, these observations support the hypothesis that the basal lamina acts as a physical barrier to HSV-1 infection of mature muscle.

Preferential adhesion to and survival on patterned laminin organizes myogenesis in vitro

We have examined a potential role for differential adhesiveness in muscle development using an in vitro model which employed the culture of myoblasts and myotubes, (conditionally immortal myogenic cells, H2k(b)-tsA58), on micropatterned surfaces. These surfaces are made up of multiple alternating tracks of hydrophobic organosilane-treated glass and untreated glass (track width ranging from 5 to 100 microm). We found that myoblasts were aligned on patterns in the presence of serum, by adhering to the tracks of untreated glass, which had preferentially adsorbed serum attachment factors. However, as serum attachment factors are not sufficient for maintenance of adhesion of mature myotubes, we determined whether precoating patterns with laminin, which maintains adhesion, could still provide a differential adhesive cue. Laminin preferentially adsorbs to the hydrophobic regions resulting in alternating tracks that have adsorbed laminin or serum attachment factors. Myoblasts were less well aligned on these patterns as they could adhere both to the untreated glass and to laminin on the previously hydrophobic tracks, but did show a preference for laminin. However, cell alignment increased upon differentiation into myotubes and continued to increase as the myotubes matured. We found that the alignment of myoblasts and myotubes on patterns increased as track width increased. In addition, adhesion to laminin was required for long term survival of the myotubes. Myotubes that had formed on nonlaminin surfaces began to detach after 2 days of differentiation. Although we found that myoblasts preferentially clustered on laminin tracks, this arrangement did not influence the diameter of the myotubes formed, upon differentiation. Instead, the number of myotubes per track increased with track width, while the myotube diameter remained constant. This uniformity of myotube diameter suggests that a mechanism exists which restricts the ability of myoblasts to undergo lateral fusion. Overall, these findings suggest that differential adhesiveness could be an important mechanism for formation and survival of myotubes, and by using these patterns we have demonstrated a mechanism controlling the formation of linear myotubes by restricting the geometry of cell-cell adhesion.

Two-step delivery of retroviruses to postmitotic, terminally differentiated cells

Recombinant replication-defective retroviral vectors are currently the most commonly used vectors for introducing foreign genes into human cells in gene therapy protocols. Their genomes stably incorporate in the host chromosomes of mitotic cells, thus ensuring stable expression. However, the applications of retroviruses to gene therapy are limited by their inability to infect postmitotic cells such as muscle fibers. In an attempt to overcome such limitations, we have developed a novel two-step transduction protocol that allows integration and expression of retroviral genes in differentiated cells. We induced DNA synthesis in terminally differentiated cultured mouse myotubes derived from both established myogenic cell lines and from primary myoblasts. We infected the postmitotic cells with a recombinant replication-defective adenoviral vector encoding the SV40 large T antigen as a mitogen. Subsequently we transduced the adenovirus-infected cells with a Moloney retroviral vector bearing the LacZ gene. Histochemical analysis revealed the coincident expression of LacZ gene in those myotubes that had been induced to synthesize DNA.

MyoD protein is differentially accumulated in fast and slow skeletal muscle fibres and required for normal fibre type balance in rodents

MyoD is a muscle-specific transcription factor involved in commitment of cells to myogenesis. MyoD mRNA levels differ between fast and slow muscles, suggesting that MyoD may regulate aspects of fibre type. Here we show that detectable MyoD protein becomes restricted during development to the nuclei of the fastest classes of fibres in fast muscles. myoDm1 mice, in which the myoD gene has been disrupted, show subtle shifts in fibre type of fast muscles toward a slower character, suggesting that MyoD is involved in the maintenance of the fast IIB/IIX fibre type. In contrast, slow muscle shifts to a faster phenotype in myoDm1. Moreover, MD6.0-lacZ transgenic mice with the myoD promoter driving lacZ, show highest beta-galactosidase activity in the fastest fibres of fast muscles, but also express low levels in slow fibres of slow, but not fast, muscles, suggesting distinct regulation of gene expression in slow fibres of fast and slow muscles.

Overexpression of the type-1 insulin-like growth factor receptor increases ligand-dependent proliferation and differentiation in bovine skeletal myogenic cultures

Previous studies demonstrated that overexpression of the type-1 insulin-like growth factor (IGF) receptor (IGF-1R) in skeletal myogenic cell lines increased proliferation and differentiation responses to IGF. However, it was unclear if such manipulations in primary, untransformed skeletal myogenic cells would result in modulation of these responses, which may be more stringently regulated in primary cells than in myogenic cell lines. In this study, low passage untransformed fetal bovine myogenic cultures were infected with a replication-deficient retroviral expression vector (LISN) coding for the human IGF-1R or with a control retroviral vector (LNL6). Bovine myogenic cultures infected with the LISN vector (Bov-LISN) displayed ten times more IGF-1Rs than controls (Bov-LNL6). Bov-LISN myogenic cultures exhibited elevated rates of IGF-I-stimulated proliferation and increased rates of terminal differentiation which were reduced to control levels by the anti-human IGF-1R antibody alpha IR3. These findings indicate overexpression of the IGF-1R can enhance IGF sensitivity and thereby modify the proliferation and differentiation behavior of untransformed low passage myoblasts. Such manipulations may be useful to increase muscle mass in clinical or agricultural applications.

Differentiation potential of conditionally immortalized mesenchymal progenitor cells from adult marrow of a H-2Kb-tsA58 transgenic mouse

Primary cultures were initiated from marrow, spleen, and bone explants of an adult H-2Kb-tsA58 transgenic mouse (immortomouse). All cultures were initiated in immortalizing conditions, and an additional marrow culture was first incubated for 1 week in standard conditions and then switched to immortalizing conditions. Marrow cells immediately immortalized were designated the marrow immediate population (MIP); those immortalized after 1 week were termed the marrow delayed population (MDP). MIP and MDP cells both contained a mixture of fibroblastic or flattened cells, and the MIP cells contained an additional subpopulation of adipocytic (Oil Red-O positive) cells. Alkaline phosphatase expression was induced by dexamethasone (10(-7) M) in MDP cells while MIP, spleen, and bone explant cells had only a low level of expression. MDP and MIP cells differentiated into bone when combined with porous calcium phosphate ceramics and implanted subcutaneously into nude mice while bone- and spleen-derived cells did not. Clones were isolated from the MDP and MIP cell populations and tested for differentiated phenotypes. Some MIP-derived clones exhibited adipocytic characteristics while MDP-derived subclones were negative. Histologic examination of porous ceramic implanted clones showed that all of the clones had osteogenic potential. Clones exposed to either dexamethasone, human recombinant bone morphogenetic protein-2, or horse serum plus hydrocortisone showed differences in expression of adipocytic or osteogenic markers. These immortalized cultures have retained both adipocytic and osteogenic potential even after 1 year of continuous culture, and provide a model system for clonal analysis of the developmental potential of marrow-derived mesenchymal precursor cells.

Successive injections in mdx mice of myoblasts grown with bFGF

We studied the effects of single and repeated sets of injections in the same muscle of mdx mice of myoblasts grown with or without a high concentration 100 ng ml-1 of basic fibroblast growth factor (bFGF). The injected myoblasts were obtained from non-dystrophic transgenic mice expressing the beta-galactosidase gene under the control of a muscle-specific promoter. In these experiments, the host muscle was not irradiated to prevent muscle regeneration by host myoblasts. The host muscle was not damaged before myoblast transplantation with notexin, marcaïne or cold to trigger a regeneration-degeneration cycle. Without such pretreatments, the first set of injections of myoblasts grown without bFGF produced only 8% beta-galactosidase-positive and dystrophin-positive muscle fibers 1 month after transplantation. The percentage of muscle fibers containing the donor reporter gene increased, however, to 26% following a second set of injections in the same muscle. The percentage of muscle fibers expressing the donor reporter gene was significantly higher when the myoblasts were grown with a high dose of bFGF. Indeed the first set of injections produced 34% beta-gal-positive fibers while a second set of injections raised this percentage to 54%. In all cases, the percentage of dystrophin-positive fibers was similar to that of beta-gal-positive fibers. Therefore a high percentage of muscle fibers of donor origin can be obtained without preliminary damaging treatments of the mdx muscle when myoblasts grown with bFGF are injected several times. The effects of bFGF is not produced by increasing the percentage of myoblasts in a primary muscle culture since improvement of myoblast transplantation was obtained with a pure myoblast clone even with a lower concentration (10 ng ml-1) of bFGF.

Partial laminin alpha2 chain restoration in alpha2 chain-deficient dy/dy mouse by primary muscle cell culture transplantation

Laminin-2 is a component of skeletal and cardiac basal lamina expressed in normal mouse and human. Laminin alpha2 chain (LAMA2), however, is absent from muscles of some congenital muscular dystrophy patients and the dystrophia muscularis (dy/dy) mouse model. LAMA2 restoration was investigated following cell transplantation in vivo in dy/dy mouse. Allogeneic primary muscle cell cultures expressing the beta-galactosidase transgene under control of a muscular promoter, or histocompatible primary muscle cell cultures, were transplanted into dy/dy mouse muscles. FK506 immunosuppression was used in noncompatible models. All transplanted animals expressed LAMA2 in these immunologically-controlled models, and the degrees of LAMA2 restoration were shown to depend on the age of the animal at transplantation, on muscle pretreatment, and on duration time after transplantation in some cases. LAMA2 did not always colocalize with new or hybrid muscle fibers formed by the fusion of donor myoblasts. LAMA2 deposition around muscle fibers was often segmental and seemed to radiate from the center to the periphery of the injection site. Allogeneic conditionally immortalized pure myogenic cells expressing the beta-galactosidase transgene were characterized in vitro and in vivo. When injected into FK506-immunosuppressed dy/dy mice, these cells formed new or hybrid muscle fibers but essentially did not express LAMA2 in vivo. These data show that partial LAMA2 restoration is achieved in LAMA2-deficient dy/dy mouse by primary muscle cell culture transplantation. However, not all myoblasts, or myoblasts alone, or the muscle fibers they form are capable of LAMA2 secretion and deposition in vivo.

Immortalization of epithelial cells

The methodologies for isolating cell lines have become very powerful, particularly in terms of retaining differentiated features of the parent cells. Cell lines can be developed from primary or early passage cells as well as from transgenic animals that carry an immortalizing gene. Cell lines from epithelia have been selected for their polar orientation, tight junction formation, and expression of differentiated markers or functions. These cell lines provide useful models for studying cell biology of specific tissues, tumorigenicity, genetic abnormalities, or to help screen for effective methods of gene therapy.

Successful histocompatible myoblast transplantation in dystrophin-deficient mdx mouse despite the production of antibodies against dystrophin

Myoblast transplantation has been considered a potential treatment for some muscular disorders. It has proven very successful, however, only in immunodeficient or immunosuppressed mice. In this study, myoblasts from C57BL10J +/+ mice were transplanted, with no immunosuppressive treatment, in the tibialis anterior of fully histocompatible but dystrophin-deficient C57BL10J mdx/mdx mice. One to 9 months after transplantation, the success of the graft was evaluated by immunohistochemistry. All the transplanted mice (n = 24) developed dystrophin-positive fibers following transplantation. Depending on myoblast cultures, transplantations, and time of analysis, the mice presented 15 to 80% of dystrophin-positive fibers in transplanted muscles. These fibers were correctly oriented and they were either from donor or hybrid origin. The dystrophin-positive fibers remained stable up to 9 months. Possible humoral and cellular immune responses were investigated after grafting. Antibodies directed against dystrophin and/or muscle membrane were developed by 58% of the mice as demonstrated by immunohistochemistry and Western blotting. Despite the presence of these antibodies, dystrophin-positive fibers were still present in grafted muscles 9 months after transplantation. Moreover, the muscles did not show massive infiltration by CD4 cells, CD8 cells, or macrophages, as already described in myoblast allotransplantations. This lack of rejection was attributed to the sequestrated nature of dystrophin after fiber formation. These results indicate that myoblast transplantation leads to fiber formation when immunocompetent but fully histocompatible donors and recipients are used and that dystrophin incompatibility alone is not sufficient to induce an immunological rejection reaction.

The H-2KbtsA58 transgenic mouse: a new tool for the rapid generation of novel cell lines

The ability to generate expanded populations of individual cell types able to undergo normal differentiation in vitro and in vivo is of critical importance in the investigation of the mechanisms that underly differentiation and in studies on the use of cell transplantation to repair damaged tissues. This review discusses the development of a strain of transgenic mice that allows the direct derivation of conditionally immortal cell lines from a variety of tissues, simply by dissociation of the tissue of interest and growth of cells in appropriate conditions. In these mice the tsA58 mutant of SV40 large T antigen is controlled by the interferon-inducible Class I antigen promoter. Cells can be grown for extended periods in vitro simply by growing them at 33 degrees C in the presence of interferon, while still retaining the capacity to undergo normal differentiation in vivo and in vitro. In addition, it appears that cell lines expressing mutant phenotypes can readily be generated by preparing cultures from appropriate offspring of matings between H-2KbtsA58 transgenic mice and mutant mice of interest.

Myoblast transfer and gene therapy in muscular dystrophies

Myoblast transfer therapy and gene therapy have both been proposed as potential treatments for inherited myopathies, such as Duchenne muscular dystrophy (DMD). The success of myoblast implantation in mouse models, where problems such as immune rejection are easily overcome, have led to similar experiments being attempted on Duchenne patients with limited, if any, success. Gene therapy, either by viral vectors or direct injection of the plasmid, has also had some success in animal models. Although both techniques, either separately or in combination, show some promise for the treatment of DMD, there are still many issues to be investigated in animal models, including the following: What is the best source of muscle precursor cells (mpc), and how may sufficient cells be obtained? What is the best vehicle for gene therapy? How far from the injection site can an implanted cell or gene have an effect? How can immune rejection of the injected cells or introduced protein be overcome? Does the introduced dystrophin lead to improved muscle function? Can cardiac muscle can be successfully treated by gene therapy? Can skeletal muscle which has undergone a great deal of damage be improved by either cell or gene therapy?

Correction of the growth defect in dwarf mice with nonautologous microencapsulated myoblasts--an alternate approach to somatic gene therapy

Most of the currently approved human gene therapy protocols depend on genetic modification of autologous cells. We propose an alternate and potentially more cost-effective approach by implanting genetically modified "universal" cell lines to deliver desired gene products to nonautologous recipients. The recombinant allogeneic cells are protected from rejection after implantation by enclosure within immuno-protective alginate-poly-L-lysine-alginate microcapsules. The clinical efficacy of this strategy is now demonstrated by implanting microencapsulated allogeneic myoblasts engineered to secrete mouse growth hormone into the growth hormone-deficient Snell dwarf mice. The treated mutants attained increases in linear growth, body weights, peripheral organ weights, and tibial growth plate thickness significantly greater than those of the untreated controls. Secondary response to the exogenous growth hormone stimulation also resulted in increased fatty acid metabolism during the first month post-implantation. The microcapsules retrieved after about 6 months of implantation appeared intact. The encapsulated myoblasts retained a viability of > 60% and continued to secrete mouse growth hormone. Thus, implantation of nonautologous recombinant cells corrected partially the pleiomorphic effects of a transcription factor mutation in the Snell dwarf mice and the encapsulated cells remained functional for at least 6 months. This simple method of delivery recombinant gene products in vivo is a benign procedure, obviates the need for patient-specific genetic modification, and is amenable to industrial-scale quality control. It should have wide applications in therapies requiring a systemic continuous supply of recombinant gene products.

Muscle: the regulation of myogenesis

The study of myogenesis in the embryo is a rapidly expanding field. In this context, the consequences of mutating different members of the MyoD family, together with an increasing number of observations that point to the importance of the MEF2 or RSRF family as myogenic regulators, and the identification of Pax-3 as a marker of early myogenic cells, have advanced our understanding of the molecular embryology of skeletal muscle. Novel cardiac regulatory factors such as Nkx-2.5 and GATA-4, in addition to MEF2 isoforms, are also beginning to be identified. At the molecular level, crystallographic studies have led to a structural model of the actinomyosin complex and also to information about how MyoD contacts DNA.

Morphometric and dynamic studies of bone changes in hyperthyroidism

Bone biopsies were performed after tetracycline double-labelling by transfixing the right iliac crest in forty hyperthyroid patients. The bone changes in cortical and trabecular bone were determined by simple measurement and point counting on decalcified and undecalcified stained sections. A slight decrease in the amount of cancellous bone was found. The mean cortical width was normal. The amount of osteoid and the length of the osteoid seams were increased, whereas the mean width of osteoid seams was decreased. The cortical osteoclastic activity and porosity were markedly increased. The trabecular osteoclasic activity was moderately increased and the mean size of periosteocytic lacunae was slightly increased. The calcification rate in cancellous bone was increased as were the active calcification surfaces (tetracycline-labelled). The osteoclastic activity in cortical bone was positively correlated to the free thyroxine index and to the urinary calcium and phosphorus excretion. The findings indicate that the bone changes in hyperthyroidism are specific and that thyroid hormone(s) stimulates both bone formation and resorption followed by increased porosity in cortical bone and by mobilization of bone mineral.