Trans-activation of the murine dystrophin gene in human-mouse hybrid myotubes

Cell Seeding Technology for Microarray-Based Quantitative Human Primary Skeletal Muscle Cell Analysis

Pipetting techniques play a crucial role in obtaining reproducible and reliable results, especially when seeding cells on small target areas, such as on microarrays, biochips or microfabricated cell culture systems. For very rare cells, such as human primary skeletal muscle cells (skMCs), manual (freehand) cell seeding techniques invariably result in nonuniform cell spreading and heterogeneous cell densities, giving rise to undesirable variations in myogenesis and differentiation. To prevent such technique-dependent variation, we have designed and fabricated a simple, low-cost pipet guidance device (PGD), and holder that works with hand-held pipettes. This work validates the accuracy and reproducibility of the PGD platform and compares its effectiveness with manual and robotic seeding techniques. The PGD system ensures reproducibility of cell seeding, comparable to that of more expensive robotic dispensing systems, resulting in a high degree of cell uniformity and homogeneous cell densities, while also enabling cell community studies. As compared to freehand pipetting, PGD-assisted seeding of C2C12 mouse myoblasts showed 5.3 times more myotube formation and likewise myotubes derived from PGD-seeded human primary skMCs were 3.6 times thicker and 2.2 times longer. These results show that this novel, yet simple PGD-assisted pipetting technique provides precise cell seeding on small targets, ensuring reproducible and reliable high-throughput cell assays.

In vitro exercise model using contractile human and mouse hybrid myotubes

Contraction of cultured myotubes with application of electric pulse stimulation (EPS) has been utilized for investigating cellular responses associated with actual contractile activity. However, cultured myotubes derived from human subjects often exhibit relatively poor EPS-evoked contractile activity, resulting in minimal contraction-inducible responses (i.e. myokine secretion). We herein describe an “in vitro exercise model”, using hybrid myotubes comprised of human myoblasts and murine C2C12 myoblasts, exhibiting vigorous contractile activity in response to EPS. Species-specific analyses including RT-PCR and the BioPlex assay allowed us to separately evaluate contraction-inducible gene expressions and myokine secretions from human and mouse constituents of hybrid myotubes. The hybrid myotubes, half of which had arisen from primary human satellite cells obtained from biopsy samples, exhibited remarkable increases in the secretions of human cytokines (myokines) including interleukins (IL-6, IL-8, IL-10, and IL16), CXC chemokines (CXCL1, CXCL2, CXCL5, CXCL6, CXCL10), CC chemokines (CCL1, CCL2, CCL7, CCL8, CCL11, CCL13, CCL16, CCL17, CCL19, CCL20, CCL21, CCL22, CCL25, CCL27), and IFN-γ in response to EPS-evoked contractile activity. Together, these results indicate that inadequacies arising from human muscle cells are effectively overcome by fusing them with murine C2C12 cells, thereby supporting the development of contractility and the resulting cellular responses of human-origin muscle cells. Our approach, using hybrid myotubes, further expands the usefulness of the “in vitro exercise model”.

A new extensively characterised conditionally immortal muscle cell-line for investigating therapeutic strategies in muscular dystrophies

A new conditionally immortal satellite cell-derived cell-line, H2K 2B4, was generated from the H2K^b-tsA58 immortomouse. Under permissive conditions H2K 2B4 cells terminally differentiate in vitro to form uniform myotubes with a myogenic protein profile comparable with freshly isolated satellite cells. Following engraftment into immunodeficient dystrophin-deficient mice, H2K 2B4 cells regenerated host muscle with donor derived myofibres that persisted for at least 24 weeks, without forming tumours. These cells were readily transfectable using both retrovirus and the non-viral transfection methods and importantly upon transplantation, were able to reconstitute the satellite cell niche with functional donor derived satellite cells. Finally using the Class II DNA transposon, Sleeping Beauty, we successfully integrated a reporter plasmid into the genome of H2K 2B4 cells without hindering the myogenic differentiation. Overall, these data suggest that H2K 2B4 cells represent a readily transfectable stable cell-line in which to investigate future stem cell based therapies for muscle disease.

The lacZ gene under the control of the 7 kb of human dystrophin muscle specific promoter is expressed in cardiac muscle but not in adult skeletal muscle in transgenic mice

In previous transgenic studies, we reported a 0.9 kb fragment from a mouse dystrophin muscle promoter that contains the regulatory elements required for expression of dystrophin only in the right heart. In this study, to further characterize the regulation of muscle type of promoter, we analyzed promoter activity and tissue specificity using a total 14 kb fragment around the human dystrophin muscular-specific exon 1 in vitro and in vivo. In vitro analysis showed that the lacZ construct of the 7 kb promoter and 7 kb intron 1 was expressed 2.5 times as strong as the lacZ construct of only the 7 kb promoter in C2/4 myotubes. In vivo analysis revealed expression of both constructs in the whole heart, skeletal muscle and vascular smooth muscle in embryos. However, in adults, the expression in skeletal muscle disappeared. We conclude that the 7 kb upstream region and the 7 kb intronic region included responsible elements for the expression in the heart, but not in skeletal muscle in vivo. It is possible that a strong enhancer element for skeletal muscle exists in some other region.

Regionalized expression of myosin isoforms in heterotypic myotubes formed from embryonic and fetal rat myoblasts in vitro

The development of mammalian limb muscles involves the appearance and fusion of at least two separate populations of muscle precursor cells. These two populations, termed embryonic and fetal myoblasts, are first detected within the limb bud at different stages of development. We have previously demonstrated that, in the rat, each myoblast population expresses a unique pattern of myosin heavy chains (MyHCs) during differentiation in vitro (Pin and Merrifield [1993] Dev. Genet. 14:356-368). Embryonic myoblasts accumulate embryonic and slow MyHCs, whereas fetal myoblasts accumulate embryonic, neonatal, and adult fast MyHCs but not slow MyHC. To determine if the two populations can fuse with each other and whether the pattern of MyHC expression is altered in the resulting heterokaryons, embryonic and fetal myoblasts were labelled with the lipophilic dye PKH26, [3H]-thymidine, or 5-bromodeoxyuridine (BRDU) and cocultured for 24-48 hr. Our results demonstrate that fusion occurs between embryonic and fetal myoblasts in vitro. Moreover, analysis of the resulting heterokaryons revealed regionalized accumulations of MyHC around individual nuclei. Interestingly, these accumulations were typical of the default pattern of expression that individual nuclei would have normally expressed in single culture. Nuclei contributed by embryonic myoblasts were surrounded by localized accumulations of slow MyHC, whereas nuclei from fetal myoblasts were surrounded by neonatal/fast MyHC. The occurrence of such nuclear domains indicates that the myoblast-specific expression of MyHC isoforms is dictated by cis-acting factors established prior to fusion.

Dystrophin-positive muscle fibers following C2 myoblast transplantation into mdx nude mice

To determine when and how the dystrophin-positive muscle fibers are formed after myoblast transplantation into dystrophin-negative muscles, the tibialis anterior (TA) muscle from mdx nude mouse was chronologically examined after C2 myoblast transplantation by immunohistochemical and glucose 6-phosphate isomerase (GPI) isoenzyme analyses. The host TA muscle transplanted with C2 myoblasts became necrotic with accumulation of basic fibroblast growth factor in the necrotic areas. This may stimulate concomitant proliferation of the host satellite cells and C2 myoblasts. Small dystrophin-positive muscle fibers appeared in the necrotic areas 3 days after transplantation. This TA muscle contained two different kinds of homodimer GPI isoenzymes but did not contain the heterodimer, suggesting rare fusion of host and donor cells. The dystrophin-positive muscle fibers in the necrotic areas rapidly increased in number and in size by 7 days, but they were smaller than the original host muscle fibers. They had central nuclei, indicating that they were regenerating fibers. The presence of heterodimer GPI isoenzyme in these muscles indicated that the regenerating fibers were mosaic host/donor muscle fibers. The dystrophin-positive muscle fibers are probably formed first by fusion of donor cells with each other and then later by the fusion of host satellite and donor cells.

Recombinant truncated dystrophin minigenes: construction, expression, and adenoviral delivery

Duchenne muscular dystrophy (DMD) is a lethal genetic disorder for which there is currently no effective treatment. Although clinical application of adenoviral vector-mediated gene transfer has not been fully developed, it shows promise for the treatment of DMD. One significant problem posed by adenoviral vector-mediated gene transfer for DMD is that currently available adenoviral vectors cannot accommodate the entire 14-kb dystrophin cDNA. To address this problem, we selectively deleted regions of the murine dystrophin cDNA to produce truncated constructs. We created three constructs, each with an in-frame deletion of a segment (3.0, 4.4, and 5.7 kb) of the spectrin-like repeat region of dystrophin. As an additional modification, we removed the majority of the 3' untranslated region of the cDNA in expression vectors encoding some of these truncated constructs. Comparative quantitative expression studies after transfection into COS and C2C12 mouse muscle cells demonstrate variations in the level of expression with different deletions in the spectrin-like repeat region. Furthermore, deletion of the 3' untranslated region was tested for one recombinant construct and resulted in a reduction in the level of expression in both cell culture systems. Toward the ultimate goal of gene transfer therapy for DMD, we created an adenoviral vector from one of our truncated constructs. Using this vector, we demonstrated truncated dystrophin expression in vitro in primary mdx (dystrophin-deficient) muscle cells and in vivo in mdx mouse muscle. In vivo, recombinant dystrophin was properly localized to the muscle membrane.

Association of aciculin with dystrophin and utrophin

Aciculin is a recently identified 60-kDa cytoskeletal protein, highly homologous to the glycolytic enzyme phosphoglucomutase type 1, (Belkin, A. M., Klimanskaya, I. V., Lukashev, M. E., Lilley, K., Critchley, D., and Koteliansky, V. E. (1994) J. Cell Sci. 107, 159-173). Aciculin expression in skeletal muscle is developmentally regulated, and this protein is particularly enriched at cell-matrix adherens junctions of muscle cells (Belkin, A. M., and Burridge, K. (1994) J. Cell Sci. 107, 1993-2003). The purpose of our study was to identify cytoskeletal protein(s) interacting with aciculin in various cell types. Using immunoprecipitation from cell lysates of metabolically labeled differentiating C2C12 muscle cells with anti-aciculin-specific antibodies, we detected a high molecular weight band (M(r) approximately 400,000), consistently coprecipitating with aciculin. We showed that this 400 kDa band comigrated with dystrophin and immunoblotted with anti-dystrophin antibodies. The association between aciculin and dystrophin in C2C12 cells was shown to resist Triton X-100 extraction and the majority of the complex could be extracted only in the presence of ionic detergents. In the reverse immunoprecipitation experiments, aciculin was detected in the precipitates with different anti-dystrophin antibodies. Immunodepletion experiments with lysates of metabolically labeled C2C12 myotubes showed that aciculin is a major dystrophin-associated protein in cultured skeletal muscle cells. Double immunostaining of differentiating and mature C2C12 myotubes with antibodies against aciculin and dystrophin revealed precise colocalization of these two cytoskeletal proteins throughout the process of myodifferentiation in culture. In skeletal muscle tissue, both proteins are concentrated at the sarcolemma and at myotendinous junctions. In contrast, utrophin, an autosomal homologue of dystrophin, was not codistributed with aciculin in muscle cell cultures and in skeletal muscle tissues. Analytical gel filtration experiments with purified aciculin and dystrophin showed interaction of these proteins in vitro, indicating that their association in skeletal muscle is due to direct binding. Whereas dystrophin was shown to be a major aciculin-associated protein in skeletal muscle, immunoblotting of anti-aciculin immunoprecipitates with antibodies against utrophin showed that aciculin is associated with utrophin in cultured A7r5 smooth muscle cells and REF52 fibroblasts. Immunodepletion experiments performed with lysates of metabolically labeled A7r5 cells demonstrated that aciculin is a major utrophin-binding protein in this cell type. Taken together, our data show that aciculin is a novel dystrophin- and utrophin-binding protein. Association of aciculin with dystrophin (utrophin) in various cell types might provide an additional cytoskeletal-matrix transmembrane link at sites where actin filaments terminate at the plasma membrane.