Human--rat muscle somatic cell hybrids form myotubes and express human muscle gene products

Hereditary human myopathies in muscle culture

In this article I illustrated the use of regenerating human muscle cultures for studying the hereditary human myopathies. Although some of the data are still controversial, they do point up the great potential of this "in vitro system". For hereditary myopathies due to developmentally regulated proteins that are expressed only at a more advanced stage of muscle differentiation, the use of highly differentiated nerve-muscle cocultures might contribute significantly to a better understanding of their developmental pathogenesis. More advanced techniques (permanent human muscle cell lines, heterokaryons, myoblast transfer, gene transfer, myogenic conversion of human non-muscle cells, cybrid clones) may provide a great deal of information at molecular level and may also have practical applications in the diagnosis or even in the treatment of hereditary human myopathies.

Thy-1-like immunoreactivity in developing chicken skeletal muscle: identification of a cross-reactive slow-fiber specific molecule that is not Thy-1

We investigated the Thy-1-like immunoreactivity during the development of chicken skeletal muscle using a group of monoclonal antibodies raised and characterised against purified chicken brain Thy-1. The immunoreactivity attributable to bona fide Thy-1 in muscle was present in nerves, connective tissue associated with the intrafusal capsule and blood vessels, and the extracellular matrix of the muscle fibres. During development there was no change in the staining of nerves, blood vessels or intrafusal capsules. However, the extracellular staining of muscle first appeared around hatching and gradually increased in intensity reaching maximal levels in the adult. The intensity of staining varied within and between the muscles examined. One of the antibodies (SB1 20.11) recognised an additional molecule that is not Thy-1 and that was localised in the cytoplasm of slow muscle fibres. This immunoreactivity was first detectable at E10 in all myotubes that contained both alkali and acid stable myosin ATPase activity (presumptive slow), but not in those myotubes with only alkali-stable myosin ATPase activity (presumptive fast). Thereafter, the staining increased to a maximum in the newly hatched animal and then decreased until reactivity was undetectable in the adult (greater than 25 weeks). All positive fibres initially stained with a uniform intensity but the time of commencement and the rate of loss of staining was variable. Those fibres that contained both acid stable and acid labile myosin ATPase activity lost the antigen much faster than the fibres containing only acid-stable myosin ATPase activity, which also tended to increase in intensity for a longer period. These may represent, respectively, the slow tonic type III fibres and the slow twitch type I fibres classified by Barnard et al. (1982).

Transfection of human skeletal muscle cells with SV40 large T antigen gene coupled to a metallothionein promoter

We have undertaken to increase the proliferative capacity of cultured human skeletal myocytes by transfection with a plasmid construct that contains the immortalizing and transforming large T antigen gene of simian virus 40 (SV40) under the control of a zinc-sensitive metallothionein promoter. This construct was chosen to permit rapid growth of transformants in zinc-containing medium, which induces high levels of T antigen expression, and muscle-specific differentiation after withdrawal of exogenous zinc, which reduces levels of T antigen. When grown in 100 microM Zn2+, transformed myocytes expressed the large T antigen, divided rapidly, and acquired an apparently unlimited proliferative capacity. Transfer of these cells to a zinc-poor medium resulted in decreased T antigen immunofluorescence, growth rate, and saturation density as well as a return to a physiological spindle morphology. Despite transformation, these cells expressed differentiation markers characteristic of myoblasts: the B isoform of creatine kinase, and surface antigens 5.1H11, D5, and Thy 1 in the presence or absence of Zn2+. When grown to high density in a serum-poor medium, these cells differentiated further into typical multinucleated myotubes that expressed the M isoform of creatine kinase and increased levels of surface antigen 5.1H11, creatine kinase, and nicotinic acetylcholine receptors, but no detectable Thy 1 antigen. The specific activity of these differentiation markers was higher when the cells were grown in the absence of added zinc. These results indicate that transformation of human skeletal myocytes with a regulatable SV40 large T antigen gene allows an increase of the proliferative capacity of these cells with preservation of their capacity to differentiate in a physiological manner.

The fusion of myoblasts

Images

Differentiation in vitro of human-mouse teratocarcinoma hybrids

The mouse embryonal carcinoma (EC) line, PCC4, was used to construct a series of somatic cell hybrids which contain a single or a few human chromosomes. The hybrids all retained the EC phenotype as determined by morphology, expression of SSEA-1, lack of cell surface H-2 antigen and cytokeratin filaments, high alkaline phosphatase levels, the ability to form EC tumors ectopically in nude mice, and the ability to differentiate in response to retinoic acid. Constitutively differentiated cloned lines were derived from retinoic acid-treated hybrid cultures. Several derived lines had a phenotype indistinguishable from that of parietal endoderm cells, which includes synthesis of large amounts of laminin, type IV procollagen, and plasminogen activator. One differentiated line showed a fibroblast-like morphology. The differentiated lines derived from two of the hybrids, MCP6 and GEOC4, stably maintained the sole human chromosomal component present in the EC progenitors. These EC hybrids therefore provide a system to study developmental regulation of the introduced and stably maintained human genetic material derived from a variety of cell types.

Differentiation in vitro of human-mouse teratocarcinoma hybrids

The mouse embryonal carcinoma (EC) line, PCC4, was used to construct a series of somatic cell hybrids which contain a single or a few human chromosomes. The hybrids all retained the EC phenotype as determined by morphology, expression of SSEA-1, lack of cell surface H-2 antigen and cytokeratin filaments, high alkaline phosphatase levels, the ability to form EC tumors ectopically in nude mice, and the ability to differentiate in response to retinoic acid. Constitutively differentiated cloned lines were derived from retinoic acid-treated hybrid cultures. Several derived lines had a phenotype indistinguishable from that of parietal endoderm cells, which includes synthesis of large amounts of laminin, type IV procollagen, and plasminogen activator. One differentiated line showed a fibroblast-like morphology. The differentiated lines derived from two of the hybrids, MCP6 and GEOC4, stably maintained the sole human chromosomal component present in the EC progenitors. These EC hybrids therefore provide a system to study developmental regulation of the introduced and stably maintained human genetic material derived from a variety of cell types.

Analysis of myogenesis by somatic cell hybridization: III. Myogenic competence of hybrids derived from rat L6 myoblasts and mouse primary fibroblasts and myoblasts

Hybrid cells derived from rat L6 myoblasts and mouse primary fibroblasts (M x F hybrids), as well as those derived from rat L6 myoblasts and mouse primary myoblasts (M x M hybrids), were examined for their ability to engage in myogenesis as judged by muscle fiber formation plus the expression of skeletal muscle myosin and creatine kinase (CK). Of 172 primary hybrid colonies scored, 59% were myogenic in the M x F fusion and 97% exhibited muscle fiber formation in the M x M fusion. Individual hybrid clones from each cross were isolated, expanded and analyzed for myogenic capabilities as well. All three M x M and all ten M x F isolated clones exhibited preferential elimination of mouse chromosomes. Nonetheless, all were capable of fusing spontaneously and of elaborating skeletal muscle myosin and CK. The three M x M hybrids expressed only MM-CK whereas nine out of ten M x F hybrids produced all three CK isoenzymes (MM, MB, BB). These results suggest that M X M hybrids express CK patterns reminiscent of the rat L6 parental cells while M X F hybrids apparently mimic mouse muscle fiber CK patterns. Various models are discussed which address these phenomena.