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

Expression of the E6 and E7 genes of human papillomavirus (HPV16) extends the life span of human myoblasts

Primary human myoblasts (satellite cells), like other human cells, have a limited life span in vitro. Here we show that expression of the E6E7 early region from human papillomavirus type 16 can greatly extend the life span of both fetal and satellite cell-derived myoblasts and release them from dependence on the growth factors normally necessary for their proliferation. Expression of either the E6 or the E7 gene alone was not sufficient to confer this phenotype, although expression of E7 did delay cellular senescence. The steady-state level of E6E7 transcripts in clonal cultures correlated with proliferative capacity and inversely with the capacity to differentiate into multinuclear myotubes. The expression of E7 alone markedly inhibited cell fusion in both adult and fetal cultures. These effects on myoblast differentiation could be related in part to the level of retinoblastoma protein (pRb), the major cellular target of E7. Terminal differentiation of skeletal myoblasts is associated with permanent withdrawal from the cell cycle; however, continued expression of E6E7 in differentiated myotubes permits reentry of myotube nuclei into S phase in response to growth factor stimulation. These results support a key role for pRb in the acquisition and maintenance of the differentiated state in human skeletal muscle and, in cooperation with p53, in the control of proliferative capacity and response to external growth factors.

Marked replicative advantage of human mtDNA carrying a point mutation that causes the MELAS encephalomyopathy

The segregation of mutant and wild-type mtDNA was investigated in transformants constructed by transferring human mitochondria from individuals belonging to four pedigrees with the MELAS encephalomyopathy-associated mtDNA mutation (MELAS is mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) into human mtDNA-less (rho 0) cells. Five of 13 clonal cell lines containing mixtures of wild-type and mutant mtDNAs were found to undergo a rapid shift of their genotype toward the pure mutant type. The other 8 cell lines, which included 6 exhibiting nearly homoplasmic mutant mtDNA, on the contrary, maintained a stable genotype. Subcloning experiments and growth rate measurements clearly indicated that an intracellular replicative advantage of mutant mtDNA was mainly responsible for the dramatic shift toward the mutant genotype observed in the unstable cell lines.

MELAS mutation in mtDNA binding site for transcription termination factor causes defects in protein synthesis and in respiration but no change in levels of upstream and downstream mature transcripts

The pathogenetic mechanism of the mitochondrial tRNA(LeuUUR) gene mutation responsible for the MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) syndrome was investigated in transformants obtained by transfer of mitochondria from three genetically unrelated MELAS patients into human mitochondrial DNA (mtDNA)-less (rho 0) cells. Marked defects in mitochondrial protein synthesis and respiratory activity were observed in transformants containing virtually pure mutant mtDNA, as compared to the parent of the rho 0 cells (the 143B cell line) or to transformants containing exclusively wild-type mtDNA, derived from one of the patients or a maternally related asymptomatic individual. A striking protective effect against the mutation was exerted in the transformants by levels of residual wild-type mtDNA above 6%. The MELAS mutation occurs within the mtDNA binding site for a protein factor (mTERF) that promotes termination of transcription at the 16S rRNA/tRNA(LeuUUR) gene boundary. A marked decrease in affinity of purified mTERF for the mutant target sequence was observed in in vitro assays. By contrast, RNA transfer hybridization experiments failed to show any significant change in the steady-state amounts of the two rRNA species, encoded upstream of the termination site, and of the mRNAs encoded downstream, in the transformants carrying the MELAS mutation.

Zinc toxicity and induction of the 72 kD heat shock protein in primary astrocyte culture

Zinc is a potent inducer of the 72 kD heat shock protein (HSP72). In brain, pathological conditions such as ischemia and seizures increase extracellular zinc. The present study examines the effect of zinc on HSP72 expression in rat primary cortical astrocyte culture. Astrocytes were grown to confluence and exposed to zinc chloride in CO2-equilibrated Earle's buffered salt solution. Expression of HSP72 was examined using immunocytochemistry. HSP72 was induced with zinc concentrations of 5 to 100 microM after 4 h exposures, or 200 to 300 microM after 15 min exposures. At the lower concentrations expression occurred in small clusters of contiguous cells. At concentrations high enough to cause cell death, HSP72-positive astrocytes formed a continuous margin around patches of dead cells. These patterns of HSP72 expression are similar to the patterns seen after cerebral ischemia in vivo. Exposure to zinc at 100 microM for 4 h or 400 microM for 15 min caused greater than 90% cell death. Increases in extracellular zinc may contribute to HSP72 induction and astrocyte death under ischemia and other pathological conditions in brain.

Conditional immortalization of normal and dysgenic mouse muscle cells by the SV40 large T antigen under the vimentin promoter control

We have created new mouse muscle cell lines of an immortalized type, expressing normal differentiation at the myotube stage: sarcomeric organization, functional excitation-contraction coupling, and triadic differentiation. The DNA immortalizing recombinant utilizes a deletion mutant of the regulatory region of the human vimentin promoter controlling the expression of a SV40 thermosensitive large T antigen, in which the small t sequence has been deleted. Skeletal mouse replicative myoblasts synthesized predominantly vimentin. After myoblast fusion the vimentin gene is strongly repressed in multinucleated syncytia. Furthermore, the normal activity of the vimentin promoter in myoblasts is increased in the large T antigen-expressing cells. We observed that continuous and rapid division of myoblasts occurs at permissive temperature, suggesting that immortalization is achieved even though the small t antigen is absent. When fusion is induced by changing media conditions, large T antigen expression is totally repressed by the vimentin promoter. When the temperature is elevated to 39 degrees C, the preexisting large T antigen is inactivated. The resulting myotubes from normal mouse differentiate totally normally as indicated by their morphology, ultrastructure, and electrophysiological properties. Mutant (muscular dysgenesis) immortalized cells express the same properties as mutant primary counterparts with no contraction, no slow Ca2+ current, and no triadic differentiation. These immortalized cell lines are potentially very useful for further pharmacology, transplantation, and cell biology studies. The vimentin promoter control of immortalizing recombinant DNA can be used for any mammalian normal and mutant muscle cell lines.

Human acid maltase-deficient myogenic cell transformation with origin-defective SV40: characterization of a cloned line

A clonal human skeletal muscle cell line showing acid maltase deficiency (AMD) was established through the transfection of origin-defective SV40 DNA. The low acid alpha-glucosidase activity and glycogenosomes in this clone corresponded to AMD. This clone, in spite of loading glycogenososmes, was competent not only as to proliferation without contact inhibition but also as to myogenic differentiation to some extent. Dexamethasone promoted the formation by the transformant of multinucleated myotubes, which expressed acetylcholine receptors. The existence of glycogenosomes did not seem to affect the proliferation or differentiation of myoblasts. The aberrant acid alpha-glucosidase expressed in the transformed myogenic clone was shown to be biochemically identical to that in AMD fibroblasts. This transformant should be of great value for investigating the pathogenesis of AMD because of the possibility of supplying semi-permanently a uniform myogenic cell line expressing AMD.

Transformation of airway epithelial cells with persistence of cystic fibrosis or normal ion transport phenotypes

These studies demonstrate the feasibility of transforming human airway epithelial cells while inducing only modest changes in function. Features central to the pathophysiology of cystic fibrosis, i.e., abnormal regulation of a chloride permeability in the apical cell membrane, appear to be preserved in the CF/T43 transformed cell line. This work indicates that additional cystic fibrosis and normal cell lines may be developed, as well as epithelial cell lines from other diseases of interest. In addition to SV40T gene, temperature-sensitive viral genes, or genes driven by inducible promoters (e.g., glucocorticoids, heavy metals) may produce cell lines in which proliferation or differentiation can be controlled. For example, the temperature-selective SV40A gene is expressed in cells cultured at the permissive (33 degrees) temperature but is degraded at the nonpermissive (40 degrees) temperature. Thus, the transfected gene may induce proliferation to increase cell number, and then be suppressed to permit expression of a differentiated phenotype. Out strategy of initially selecting clones by G418 resistance and then selecting clones that develop functional tight junctions (and a transepithelial resistance) was useful in identifying a cell line with highly differentiated phenotypic properties. Cell lines that do not form transepithelial resistances may be valuable for studies that do not depend on cell polarization. Initial evidence suggests that some of the differentiated properties of CF/T43, i.e., formation of functional tight junctions and a transepithelial resistance, are lost at late passages.(ABSTRACT TRUNCATED AT 250 WORDS)

Generation of human myogenic cell lines by the transformation of primary culture with origin-defective SV40 DNA

The gene transfection technique was applied to establish clonal human skeletal muscle cell lines. DNA of a replication origin-defective mutant of SV40 was transfected into a primary culture of human skeletal muscle by the DNA-calcium phosphate co-precipitation method, and myoblast-derived cells were selected from among the transformed cells and cloned. The myogenic clonal cells exhibited an enhanced growth rate and an unlimited life span, which indicated that a stable supply of a large quantity of cultured human myogenic cells without contaminating fibroblasts was possible. In addition, despite the transformation, the transformed clones retained a certain differentiation ability, that is, they could form multinucleated cells or express a muscle-specific isomer of creatine kinase. These characteristics of transformed myogenic cells should be of great value in studies on the molecular pathologies of various myopathies.