The correlation between the synthesis of skeletal muscle actin, myosin heavy chain, and myosin light chain and the accumulation of corresponding mRNA sequences during myogenesis

Sex Dimorphism in Muscle Damage-induced Inflammation

INTRODUCTION

The purpose of this study was to determine the effect of resistance exercise (RE)-induced hormonal changes on intramuscular cytokine gene expression after muscle damage in untrained men and women.

METHODS

Men (n = 8, 22 ± 3 yr) and women (n = 8, 19 ± 1 yr) completed two sessions of 80 unilateral maximal eccentric knee extensions followed by either an upper body RE bout (EX) or a time-matched period (CON). Muscle samples (vastus laterals) were analyzed for mRNA expression of interleukin (IL) 6, IL-10, IL-15, TNFA, TGFB, CCL2, and CD68 at PRE, 12 h, and 24 h after the session.

RESULTS

A significant time-sex-condition interaction was found for TGFB with an increase for EX in men at 12 h from PRE. For EX, TGFB was also greater in men than in women at 12 and 24 h. Significant time-sex and condition-sex interactions were found for IL-10 with an increase for men that was greater than for women at 12 and 24 h. IL-10 was lower in EX than CON for men. A significant time-sex interaction was found for TNFA with an increase for men that was greater than for women at 24 h. A significant time-condition interaction was found for CD68 with an increase at 12 h and decrease at 24 h for EX and CON. CD68 was lower in EX than CON at 12 h. A significant time effect was found for IL6 and CCL2 with an increase at 12 and 24 h.

CONCLUSIONS

Results suggest that women seem to have a muted intramuscular cytokine (i.e., IL-10, TNF-α, and TGF-β) response to muscle damage compared with men.

Farewell to Professor David Yaffe - A pillar of the myogenesis field

It is with great sadness that we have learned about the passing of Professor David Yaffe (1929-2020, Israel). Yehi Zichro Baruch - May his memory be a blessing. David was a man of family, science and nature. A native of Israel, David grew up in the historic years that preceded the birth of the State of Israel. He was a member of the group that established Kibbutz Revivim in the Negev desert, and in 1948 participated in Israel's War of Independence. David and Ruth eventually joined Kibbutz Givat Brenner by Rehovot, permitting David to be both a kibbutz member and a life-long researcher at the Weizmann Institute of Science, where David received his PhD in 1959. David returned to the Institute after his postdoc at Stanford. Here, after several years of researching a number of tissues as models for studying the process of differentiation, David entered the myogenesis field and stayed with it to his last day. With his dedication to the field of myogenesis and his commitment to furthering the understanding of the People and the Land of Israel throughout the international scientific community, David organized the first ever myogenesis meeting that took place in Shoresh, Israel in 1975. This was followed by the 1980 myogenesis meeting at the same place and many more outstanding meetings, all of which brought together myogenesis, nature and scenery. Herein, through the preparation and publication of this current manuscript, we are meeting once again at a "David Yaffe myogenesis meeting". Some of us have been members of the Yaffe lab, some of us have known David as his national and international colleagues in the myology field. One of our contributors has also known (and communicates here) about David Yaffe's earlier years as a kibbutznick in the Negev. Our collective reflections are a tribute to Professor David Yaffe. We are fortunate that the European Journal of Translational Myology has provided us with tremendous input and a platform for holding this 2020 distance meeting "Farwell to Professor David Yaffe - A Pillar of the Myogenesis Field".

Diverse roles of the actin cytoskeleton in striated muscle

In addition to the highly specialized contractile apparatus, it is becoming increasingly clear that there is an extensive actin cytoskeleton which underpins a wide range of functions in striated muscle. Isoforms of cytoskeletal actin and actin-associated proteins (non-muscle myosins, cytoskeletal tropomyosins, and cytoskeletal alpha-actinins) have been detected in a number of regions of striated muscle: the sub-sarcolemmal costamere, the Z-disc and the T-tubule/sarcoplasmic reticulum membranes. As the only known function of these proteins is through association with actin filaments, their presence in striated muscles indicates that there are spatially and functionally distinct cytoskeletal actin filament systems in these tissues. These filaments are likely to have important roles in mechanical support, ion channel function, myofibrillogenenous and vesicle trafficking.

Differentiation-dependent PTPIP51 expression in human skeletal muscle cell culture

Protein tyrosine phosphatase-interacting protein 51 (PTPIP51) expression was analyzed in proliferating and differentiating human myogenic cells cultured in vitro. Satellite cell cultures derived from four different individuals were used in this study. To analyze the expression of PTPIP51, myoblasts were cultured under conditions promoting either proliferation or differentiation. In addition, further differentiation of already-differentiated myobtubes was inhibited by resubmitting the cells to conditions promoting proliferation. PTPIP51 protein and mRNA were investigated in samples taken at defined time intervals by immunostaining, immunoblotting, in situ hybridization, and PCR. Image analyses of fluorescence immunostainings were used to quantify PTPIP51 in cultured myoblasts and myotubes. Myoblasts grown in the presence of epidermal and fibroblast growth factors (EGF and FGF), both promoting proliferation, expressed PTPIP51 on a basic level. Differentiation to multinuclear myotubes displayed a linear increase in PTPIP51 expression. The rise in PTPIP51 protein was paralleled by an augmented expression of muscle-specific proteins, namely, sarcoplasmic reticulum Ca(2+) ATPase and myosin heavy-chain protein, both linked to a progressive state of myotubal differentiation. This differentiation-induced increase in PTPIP51 was partly reversible by resubmission of differentiated myotubes to conditions boosting proliferation. The results clearly point toward a strong association between PTPIP51 expression and differentiation in human muscle cells.

Reinvestigation of the dysbindin subunit of BLOC-1 (biogenesis of lysosome-related organelles complex-1) as a dystrobrevin-binding protein

Dysbindin was identified as a dystrobrevin-binding protein potentially involved in the pathogenesis of muscular dystrophy. Subsequently, genetic studies have implicated variants of the human dysbindin-encoding gene, DTNBP1, in the pathogeneses of Hermansky-Pudlak syndrome and schizophrenia. The protein is a stable component of a multisubunit complex termed BLOC-1 (biogenesis of lysosome-related organelles complex-1). In the present study, the significance of the dystrobrevin-dysbindin interaction for BLOC-1 function was examined. Yeast two-hybrid analyses, and binding assays using recombinant proteins, demonstrated direct interaction involving coiled-coil-forming regions in both dysbindin and the dystrobrevins. However, recombinant proteins bearing the coiled-coil-forming regions of the dystrobrevins failed to bind endogenous BLOC-1 from HeLa cells or mouse brain or muscle, under conditions in which they bound the Dp71 isoform of dystrophin. Immunoprecipitation of endogenous dysbindin from brain or muscle resulted in robust co-immunoprecipitation of the pallidin subunit of BLOC-1 but no specific co-immunoprecipitation of dystrobrevin isoforms. Within BLOC-1, dysbindin is engaged in interactions with three other subunits, named pallidin, snapin and muted. We herein provide evidence that the same 69-residue region of dysbindin that is sufficient for dystrobrevin binding in vitro also contains the binding sites for pallidin and snapin, and at least part of the muted-binding interface. Functional, histological and immunohistochemical analyses failed to detect any sign of muscle pathology in BLOC-1-deficient, homozygous pallid mice. Taken together, these results suggest that dysbindin assembled into BLOC-1 is not a physiological binding partner of the dystrobrevins, likely due to engagement of its dystrobrevin-binding region in interactions with other subunits.

A novel role for non-muscle gamma-actin in skeletal muscle sarcomere assembly

Existing models describing sarcomere assembly have arisen primarily from studies using cardiac muscle. In contrast to cardiac muscle, skeletal muscle differentiation is characterised by dramatic changes in protein expression, from non-muscle to muscle-specific isoforms before organisation of the sarcomeres. Consequently, little is understood of the potential influence of non-muscle cytoskeletal proteins on skeletal sarcomere assembly. To address this issue, transfectant (gamma33-B1) and control mouse C2 myoblasts were differentiated to form myotubes, and various stages of skeletal sarcomere assembly were studied. Organisation of non-muscle gamma-actin and co-localisation with sarcomeric alpha-actinin, an early marker of sarcomere assembly and a major component of Z lines, was noted. gamma-Actin was also identified in young myotubes with developing sarcomeric myofibrils in regenerating adult mouse muscle. Localisation of gamma-actin in a different area of the myotube to the muscle-specific sarcomeric alpha-actin also indicated a distinct role for gamma-actin. The effects of aberrant gamma-actin expression in other myoblast lines, further suggested a sequestering role for gamma-actin. These observations make the novel suggestion that non-muscle gamma-actin plays a role in skeletal sarcomere assembly both in vitro and in vivo. Consequently, a modified model is proposed which describes the role of gamma-actin in skeletal sarcomere assembly.

beta- and gamma-actin genes differ in their mechanisms of down-regulation during myogenesis

During the differentiation of myoblasts to form myotubes, the expression patterns of the different actin isoforms change. The cytoplasmic actins, beta and gamma, are down-regulated and the muscle specific isoforms are up-regulated. The region responsible for the down-regulation of the beta-actin gene has been located in the 3'end of the gene. Since the beta- and gamma-actin genes arose from a gene duplication (Erba et al. [1988] J. Cell. Biol. 8:1775-1789), it is possible that the region responsible for down-regulation of the gamma-actin gene may also be in the 3'end of the gene. We have tested this by transfection of human gamma-actin gene constructs into myogenic C2 cells. To our surprise, we found that the region responsible for down-regulation of the gamma-actin gene during differentiation is not in the 3' end of the gene in contrast to that for beta-actin. Rather, we found that intron III is required for appropriate down-regulation of gamma-actin during myogenesis. Intron III containing transcripts from the gamma-actin gene were also found to accumulate during myogenesis. We, therefore, propose that excision of intron III from the primary transcript is inhibited during myogenesis resulting in degradation of the RNA. Removal of intron III from the gene allows it to escape this regulatory mechanism.

Involvement of gap junctional communication in myogenesis

Cell-to-cell communication plays important roles in development and in tissue morphogenesis. Gap junctional intercellular communication (GJIC) has been implicated in embryonic development of various tissues and provides a pathway to exchange ions, secondary messengers, and metabolites through the intercellular gap junction channels. Although GJIC is absent in adult skeletal muscles, the formation of skeletal muscles involves a sequence of complex events including cell-cell interaction processes where myogenic cells closely adhere to each other. Much experimental evidence has shown that myogenic precursors and developing muscle fibers can directly communicate through junctional channels. This review summarizes current knowledge on the GJIC and developmental events involved in the formation of skeletal muscle fibers and describes recent progress in the investigation of the role of GJIC in myogenesis: evidence of gap junctions in somitic and myotomal tissue as well as in developing muscle fibers in situ, GJIC between perfusion myoblasts in culture, and involvement of GJIC in cytodifferentiation of skeletal muscle cells and in myoblast fusion. A model of intercellular signaling is proposed where GJIC participates to coordinate a multicellular population of interacting myogenic precursors to allow commitment to the skeletal muscle fate.

Regulation of myofibrillar protein turnover during maturation in normal and undernourished rat pups

The study tested the hypothesis that a higher rate of myofibrillar than sarcoplasmic protein synthesis is responsible for the rapid postdifferentiation accumulation of myofibrils and that an inadequate nutrient intake will compromise primarily myofibrillar protein synthesis. Myofibrillar (total and individual) and sarcoplasmic protein synthesis, accretion, and degradation rates were measured in vivo in well-nourished (C) rat pups at 6, 15, and 28 days of age and compared at 6 and 15 days of age with pups undernourished (UN) from birth. In 6-day-old C pups, a higher myofibrillar than sarcoplasmic protein synthesis rate accounted for the greater deposition of myofibrillar than sarcoplasmic proteins. The fractional synthesis rates of both protein compartments decreased with age, but to a greater degree for myofibrillar proteins (-54 vs. -42%). These decreases in synthesis rates were partially offset by reductions in degradation rates, and from 15 days, myofibrillar and sarcoplasmic proteins were deposited in constant proportion to one another. Undernutrition reduced both myofibrillar and sarcoplasmic protein synthesis rates, and the effect was greater at 6 (-25%) than 15 days (-15%). Decreases in their respective degradation rates minimized the effect of undernutrition on sarcoplasmic protein accretion from 4 to 8 days and on myofibrillar proteins from 13 to 17 days. Although these adaptations in protein turnover reduced overall growth of muscle mass, they mitigated the effects of undernutrition on the normal maturational changes in myofibrillar protein concentration.

Calpain and calpastatin in myoblast differentiation and fusion: effects of inhibitors

Myoblast differentiation and fusion to multinucleated muscle cells can be studied in myoblasts grown in culture. Calpain (Ca(2+)-activated thiol protease) induced proteolysis has been suggested to play a role in myoblast fusion. We previously showed that calpastatin (the endogenous inhibitor of calpain) plays a role in cell membrane fusion. Using the red cell as a model, we found that red cell fusion required calpain activation and that fusibility depended on the ratio of cell calpain to calpastatin. We found recently that calpastatin diminishes markedly in myoblasts during myoblast differentiation just prior to the start of fusion, allowing calpain activation at that stage; calpastatin reappears at a later stage (myotube formation). In the present study, the myoblast fusion inhibitors TGF-beta, EGTA and calpeptin (an inhibitor of cysteine proteases) were used to probe the relation of calpastatin to myoblast fusion. Rat L8 myoblasts were induced to differentiate and fuse in serum-poor medium containing insulin. TGF-beta and EGTA prevented the diminution of calpastatin. Calpeptin inhibited fusion without preventing diminution of calpastatin, by inhibiting calpain activity directly. Protein levels of mu-calpain and m-calpain did not change significantly in fusing myoblasts, nor in the inhibited, non-fusing myoblasts. The results indicate that calpastatin level is modulated by certain growth and differentiation factors and that its continuous presence results in the inhibition of myoblast fusion.

The regulation of alpha 5 beta 1 integrin expression in human muscle cells

The expression of alpha 5 beta 1 integrin was examined in either cloned or fluorescence-activated cell-sorted satellite cells derived from human biceps muscle. Removal of serum and factors required for muscle cell growth and proliferation both induced terminal differentiation and resulted in a coordinate downregulation of mRNA transcripts encoding alpha 5 and beta 1 integrin subunits. A corresponding downregulation of the alpha 5 subunit occurred at the protein level. Treatment of cultures with 5-bromo-2'-deoxyuridine (BUdR), a thymidine analog which inhibits muscle cell differentiation, resulted in increased expression of alpha 5 integrin subunit at both the mRNA and protein levels. However, levels of alpha 5 subunit message and protein were still markedly downregulated on removal of serum and growth factors from BUdR-treated cultures, indicating that downregulation of alpha 5 beta 1 integrin during myogenesis does not require and is not a consequence of muscle cell terminal differentiation. Downregulation of alpha 5 integrin subunit expression could be prevented by maintenance of cells in medium supplemented with serum and growth factors, although no single defined component of this medium could on its own prevent the downregulation of alpha 5 integrin subunit expression. Collectively, these results suggest that downregulation of alpha 5 beta 1 integrin expression is not a consequence of muscle cell terminal differentiation, but is dependent on a combination of exogenous growth factors which are also required for muscle cell growth and proliferation.

Cloning of alpha 2 chain of type VI collagen and expression during mouse development

We have previously described the molecular cloning of a cDNA probe which detects a 6 kb mRNA termed pOb24. pOb24 mRNA appeared to be a marker of the preadipose state both in vitro and in vivo. A pOb24 genomic fragment was isolated and used to screen cDNA libraries in order to isolate the full-length pOb24 cDNA and to identify the corresponding protein. The screening yielded a new cDNA clone which detected a 3.7 kb mRNA species in addition to the 6 kb mRNA species. Sequences at the 3' end of the 6 kb and 3.7 kb mRNAs indicate that both mRNAs are generated from the same gene through the use of two different polyadenylation sites. The protein encoded by the 3.7 kb mRNA appeared to be homologous to the human alpha 2 chain of type VI collagen (A2COL6). The expression of the A2COL6 gene was not confined to adipose tissue; mRNA species can be detected in ovaries, adrenal glands and lungs but not in liver and skeletal muscle. The expression appeared specific for initial phase(s) of cell differentiation since it is parallel to that of the MyoD1 gene during muscle embryogenesis in vivo. In the myogenic C2C12 cell line, the A2COL6 gene exhibited the same regulation as MyoD1 and myogenin genes. These results indicate that A2COL6 gene expression is a marker of the preadipose state, but may also be a marker of other differentiation programmes such as that of muscle.

High level expression of transfected beta- and gamma-actin genes differentially impacts on myoblast cytoarchitecture

The impact of the human beta- and gamma-actin genes on myoblast cytoarchitecture was examined by their stable transfection into mouse C2 myoblasts. Transfectant C2 clones expressing high levels of human beta-actin displayed increases in cell surface area. In contrast, C2 clones with high levels of human gamma-actin expression showed decreases in cell surface area. The changes in cell morphology were accompanied by changes in actin stress-fiber organization. The beta-actin transfectants displayed well-defined filamentous organization of actin; whereas the gamma-actin transfectants displayed a more diffuse organization of the actin cables. The role of the beta-actin protein in generating the enlarged cell phenotype was examined by transfecting a mutant form of the human beta-actin gene. Transfectant cells were shown to incorporate the aberrant actin protein into stress-fiber-like structures. High level expression of the mutant beta-actin produced decreases in cell surface area and disruption of the actin microfilament network similar to that seen with transfection of the gamma-actin gene. In contrast, transfection of another mutant form of the beta-actin gene which encodes an unstable protein had no impact on cell morphology or cytoarchitecture. These results strongly suggest that it is the nature of the encoded protein that determines the morphological response of the cell. We conclude that the relative gene expression of beta- and gamma-actin is of relevance to the control of myoblast cytoarchitecture. In particular, we conclude that the beta- and gamma-actin genes encode functionally distinct cytoarchitectural information.

Coordination of skeletal muscle gene expression occurs late in mammalian development

The acquisition of specialized skeletal muscle fiber phenotypes during development is investigated by systematic measurement of the accumulation of 21 contractile protein mRNAs during hindlimb development in the rat and the human. During early myotube formation in both species there is no coordination of expression of either fast or slow contractile protein isoform genes, but rather some slow, some fast, and some cardiac isoforms are expressed. Some isoforms are not detected at all in early myotubes. From Embryonic Day 19 in the rat, and after 14 weeks in the human, a strong bias toward fast isoform expression is evident for all gene families examined. This results in the establishment of a coordinated fast isoform phenotype at birth in the rat, and by 24 weeks in the human fetus. Unexpectedly, during secondary myotube formation in the rat we observe sudden rises and falls in contractile protein gene output. We interpret these fluctuations in terms of periods of myoblast proliferation followed by synchronized fusion into myotubes. The data presented indicate that each contractile protein gene has its own determinants of mRNA accumulation and that the different myoblast populations which contribute to the developing limb are not intrinsically programmed to produce particular coordinated phenotypes with respect to the non-myosin heavy chain contractile proteins.

Regulation of contractile protein gene family mRNA pool sizes during myogenesis

During myogenesis, muscle contractile protein gene expression is induced and the products are used to assemble the contractile apparatus characteristic of striated muscle. The different muscle proteins are accumulated in a fixed stoichiometric ratio related to their organization in the contractile apparatus. We have examined the relationship between contractile protein gene expression and the maintenance of stoichiometry at different stages of human myogenesis. Essentially all of the known components of adult human skeletal muscle thick and thin filaments have been cloned in the form of cDNAs and used to generate isoform-specific DNA probes. The expression of fast, slow, and cardiac isoforms was measured in human myogenic primary culture and in fetal and adult human skeletal muscle. We observed that neither fast nor slow nor cardiac isoforms are coordinately regulated at the level of comparative transcript accumulation throughout myogenesis. Thus, the stoichiometry of contractile protein levels cannot be explained by coordination of expression in each of these isoform classes. However, we find that the stoichiometry of mRNA accumulation of each gene family is very similar among three developmental stages: myotubes, fetal skeletal muscle, and adult skeletal muscle. This is consistent with the possibility that the maintenance of stoichiometry between the contractile proteins could be largely regulated by the total accumulation of mRNA from each of these gene families.

Coexpression of alpha-sarcomeric actin, alpha-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.

Differentiation-dependent expression of apolipoprotein A-I in chicken myogenic cells in culture

Northern blot hybridization experiments showed that Apolipoprotein A-I (Apo A-I) mRNA is present at high concentration in chicken myotubes cultured in vitro, while it is virtually absent in fibroblasts and myoblasts. Myotubes are also capable of translating and secreting in the culture medium a protein which is specifically immunoprecipitated by anti-Apo A-I antibodies and has the same electrophoretic mobility as Apo A-I purified from circulating high-density lipoproteins. The appearance of Apo A-I mRNA in myotubes depends on the transcriptional activation of the corresponding gene, as it was shown by hybridizing 32P-labeled RNA synthesized in isolated nuclei to Apo A-I cDNA. The activation of the Apo A-I gene is regulated by the muscle cell coordinately with muscle-specific genes. In fact, treatment with TPA, a powerful inhibitor of differentiation, efficiently prevents myoblasts from producing Apo A-I mRNA, as well as muscle actin mRNA, and causes myotubes to quickly cease Apo A-I mRNA synthesis. The existence of a strict relationship between Apo A-I mRNA concentration and myogenic cell differentiation was also confirmed by experiments with quail myoblasts transformed with a temperature-sensitive mutant of the Rous Sarcoma Virus. Cells raised at the permissive temperature (undifferentiated phenotype) do not contain Apo A-I as well as alpha-actin mRNAs, while shifting to the nonpermissive temperature (differentiated phenotype) causes a rapid increase in Apo A-I and alpha-actin mRNA concentration.

The amount of the endogenous and exogenous skeletal muscle actin mRNA in the heart of transgenic mice is affected by the genotype of the cardiac actin gene

Both skeletal muscle and cardiac actins are co-expressed in the newborn heart. However, the amount of the skeletal muscle actin and its mRNA rapidly decreases during early development and the cardiac actin predominates in the adult heart. In BALB/c and DBA mice there is a mutation in the cardiac actin gene which is associated with decreased levels of cardiac actin mRNA and high levels of the skeletal muscle actin transcript in the adult heart. To examine the possibility that the amount of cardiac actin gene product modulates the expression of the skeletal muscle actin gene in the heart, transgenic mice carrying a tagged skeletal muscle actin gene were produced, and the expression of the endogenous and endogenous and exogenous actin gene was analyzed in offspring carrying different combinations of the cardiac actin alleles. It was found that both the endogenous and exogenous skeletal muscle actin genes were expressed at low levels in the heart of adult mice homozygous for the wild-type cardiac actin gene allele, at abnormally high levels in mice homozygous for the mutated cardiac actin allele, and at intermediate levels in heterozygous mice. This shows that the level of expression of the cardiac actin gene has a trans effect on the expression of the skeletal muscle actin gene.

Transcriptional regulation of actin and myosin genes during differentiation of a mouse muscle cell line

During terminal differentiation of skeletal muscle cells in vitro there is a transition from a predominantly nonmuscle contractile protein phenotype to a sarcomeric contractile protein phenotype. In order to investigate whether this transition and subsequent changes in expression are primarily transcriptionally regulated, we have analysed the rate of transcription and level of corresponding RNA accumulation of actin and myosin light chain genes during differentiation of a mouse muscle cell line under different culture conditions (low-serum and serum-free). We have found by 'nuclear run-on' analysis, that the alpha-cardiac actin, alpha-skeletal actin, myosin light chain 1F/3F and embryonic myosin light chain genes are transcriptionally activated as myoblasts begin to fuse to form myotubes. In contrast the nonsarcomeric beta-actin gene is transcribed at high levels in myoblasts and is transcriptionally down-regulated during differentiation. There is a sequential transition in transcription and RNA accumulation from predominantly alpha-cardiac to predominantly alpha-skeletal actin during subsequent myotube maturation, which reflects the pattern of expression found during development in vivo. A similar transition from embryonic to adult patterns of myosin light chain expression does not occur. RNA accumulation of actin and myosin light chains is regulated at both transcriptional and post-transcriptional levels. In our culture system the expression of myosin light chains 1F and 3F, which are encoded by a single gene, is uncoupled, 3F predominating. These data are discussed in the context of gene regulation mechanisms.

Progress in the recognition and treatment of soft tissue sarcomas

Over the past 10 years there has been significant progress made in the recognition and treatment of soft tissue sarcomas. With the advent of CAT scans and MRI, preoperative delineation of soft tissue tumors has become readily available. The diagnostic use of these modalities in patients presenting with an ill-defined asymptomatic mass has been extremely helpful in terms of screening patients to decide whether or not a biopsy is indicated. These techniques have also provided a much clearer delineation of the anatomic extent of the primary tumor, which has been of great assistance both in radiation therapy treatment calculations as well as in preoperative surgical planning. The recognition that tumor grade is the dominant prognostic variable has resulted in the more common use of a grading system, and a more uniform reporting and stratification of end results. Recent studies with immunohistochemical staining have proven of value in determining the histogenesis of many tumors that in the past were difficult to classify accurately. Most recently the use of flow cytometry suggests that this will also be a valuable adjunct in determining tumor grade and thereby prognosis. The most recent investigations of molecular biologic evaluation of genetic DNA and RNA sequences, as well as of oncogenes are extremely interesting from a diagnostic standpoint and in demonstrating the potential of molecular biologic evaluation for understanding the origin of these tumors. Multimodality therapy with surgery, radiation, chemotherapy, or all three has resulted in a marked improvement in local tumor control for patients with soft tissue sarcomas. The combination of modalities has allowed smaller surgical excisions of the tumor and thereby preservation of the extremity and much of its function. There are currently several different methods of multimodality therapy used including neoadjuvant therapy and postoperative therapy, both of which have been proven efficacious. Chemotherapy is now playing an increased role in clinical investigation and treatment. The availability of Adriamycin, DTIC, cisplatin, and most recently ifosfamide has added several chemotherapeutic agents for use by the clinician. Combination chemotherapy and radiation is of value in the neoadjuvant setting, and several studies are now underway to determine whether postoperative adjuvant chemotherapy is of similar value in reducing systemic spread of disease. Finally, surgical resection of pulmonary metastases has been proven of value in 20% to 25% of patients who subsequently develop metastatic disease. As a result of these advances in several different treatment disciplines, the overall survival rate and quality of life of patients with soft tissue sarcoma have improved markedly over the past 10 years.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative effects of hindlimb suspension and exercise on skeletal muscle myosin isozymes in rats

The purpose of this study was to ascertain the time course of changes, whilst suspending the hindlimb and physical exercise training, of myosin light chain (LC) isoform expression in rat soleus and vastus lateralis muscles. Two groups of six rats were suspended by their tails for 1 or 2 weeks, two other groups of ten rats each were subjected to exercise training on a treadmill for 9 weeks, one to an endurance training programme (1-h running at 20 m.min-1 5 days.week-1), and the other to a sprint programme (30-s bouts of running at 60 m.min-1 with rest periods of 5 min). At the end of these experimental procedures, soleus and vastus lateralis superficialis muscles were removed for myosin LC isoform determination by two-dimensional gel electrophoresis. Hindlimb suspension for 2 weeks significantly increased the proportion of fast myosin LC and decreased slow myosin LC expression in the soleus muscle. The pattern of myosin LC was unchanged in the vastus lateralis muscle. Sprint training or endurance training for 9 weeks increased the percentage of slow myosin LC in vastus lateralis muscle, whereas soleus muscle myosin LC was not modified. These data indicate that hindlimb suspension influences myosin LC expression in postural muscle, whereas physical training acts essentially on phasic muscle. There were no differences in myosin LC observed under the influence of sprint- or endurance-training programme.

Soft tissue sarcomas: current trends in diagnosis and management

Sarcomas (fleshy tumors) were distinguished from carcinoma (crab leg tumors) at the time of Hippocrates. Sarcomas are related embryologically to leukemias and lymphomas because all are thought to be malignancies of mesodermal embryologic origin. Neurosarcomas, however, are an exception, since they arise from tissue of neuroepidermal origin. Malignant sarcomas of the soft tissue represent an unusual primary malignant tumor. These lesions are characterized by their diversity in the histologic appearance and in their biologic behavior, as well as in anatomical origin. Currently there are approximately 7,400 newly diagnosed bone and soft tissue sarcomas, and approximately 4,200 deaths per year in the United States.1 The incidence of sarcomas varies by histologic type in various age groups. Embryonal rhabdomyosarcoma in the orbit peaks in the 4-year-old, and in the urinary tract in adolescence.2 Osteosarcoma has peak incidence in the teenage years, and Ewing's sarcoma develops between the ages of 15 and 30. Other sarcomas such as malignant fibrous histiocytoma and chondrosarcoma generally occur in patients aged more than 55 years. The incidence of osteosarcoma in whites and nonwhites is equal; Ewing's sarcoma is predominantly a disease of Caucasians.3 The relatively infrequent occurrence of these tumors plus their diverse histology and diverse presentations have made it difficult for any one institution to have enough patients to directly compare, in a randomized prospective fashion, one treatment with another in order to determine the optimal primary therapy. The purpose of this monograph is to review recent concepts in terms of pathology, surgery, radiation therapy, chemotherapy, and multimodality therapy.

The ski oncogene induces muscle differentiation in quail embryo cells

Quail embryo cells (QECs) are primary cultures of fibroblastoid cells that become myogenic after infection with avian retroviruses expressing the ski oncogene (SKVs). ski also stimulates proliferation of QECs and induces morphological transformation and anchorage-independent growth. Paradoxically, ski-transformed clones picked from soft agar are capable of muscle differentiation. ski-induced differentiation is essentially indistinguishable from that of uninfected myoblasts in culture with regard to muscle-specific gene expression, commitment, and inhibition by growth factors or other oncogenes. However, ski-induced myoblasts have less stringent requirements for growth and differentiation. Uninfected QECs cannot differentiate and do not express an early marker for the myogenic lineage. Clonal analysis indicates that at least 40% of QECs are converted by ski to differentiating myoblasts. The data suggest that ski induces either the capacity for differentiation in an "incompetent" muscle precursor or the determination of nonmyogenic cells to the myogenic lineage.

Cross-binding of factors to functionally different promoter elements in c-fos and skeletal actin genes

A conserved 28-base-pair element in the skeletal actin promoter was sufficient to activate muscle-specific expression when placed upstream of a TATA element. This muscle regulatory element (MRE) is similar in structure to the serum response element (SRE), which is present in the promoters of the c-fos proto-oncogene and the nonmuscle actin genes. The SRE can function as a constitutive promoter element. Though the MRE and SRE differed in their tissue-specific expression properties, both elements bound to the same protein factors in vitro. These proteins are the serum response factor (SRF) and the muscle actin promoter factors 1 and 2 (MAPF1 and MAPF2). The SRF and MAPF proteins were resolved by chromatographic procedures, and they differed in their relative affinities for each element. The factors were further distinguished by their distinct, but overlapping, methylation interference footprint patterns on each element. These data indicate that the differences in tissue-specific expression may be due to a complex interaction of protein factors with these sequences.

Fast skeletal muscle-specific expression of a quail troponin I gene in transgenic mice

We have produced seven lines of transgenic mice carrying the quail gene encoding the fast skeletal muscle-specific isoform of troponin I (TnIf). The quail DNA included the entire TnIf gene, 530 base pairs of 5'-flanking DNA, and 1.5 kilobase pairs of 3'-flanking DNA. In all seven transgenic lines, normally initiated and processed quail TnIf mRNA was expressed in skeletal muscle, where it accumulated to levels comparable to that in quail muscle. Moreover, in the three lines tested, quail TnIf mRNA levels were manyfold higher in a fast skeletal muscle (gastrocnemius) than in a slow skeletal muscle (soleus). We conclude that the cellular mechanisms directing muscle fiber type-specific TnIf gene expression are mediated by cis-regulatory elements present on the introduced quail DNA fragment and that they control TnIf expression by affecting the accumulation of TnIf mRNA. These elements have been functionally conserved since the evolutionary divergence of birds and mammals, despite the major physiological and morphological differences existing between avian (tonic) and mammalian (twitch) slow muscles. In lines of transgenic mice carrying multiple tandemly repeated copies of the transgene, an aberrant quail TnIf transcript (differing from normal TnIf mRNA upstream of exon 2) also accumulated in certain tissues, particularly lung, brain, spleen, and heart tissues. However, this aberrant transcript was not detected in a transgenic line which carries only a single copy of the quail gene.

Cross-binding of factors to functionally different promoter elements in c-fos and skeletal actin genes

A conserved 28-base-pair element in the skeletal actin promoter was sufficient to activate muscle-specific expression when placed upstream of a TATA element. This muscle regulatory element (MRE) is similar in structure to the serum response element (SRE), which is present in the promoters of the c-fos proto-oncogene and the nonmuscle actin genes. The SRE can function as a constitutive promoter element. Though the MRE and SRE differed in their tissue-specific expression properties, both elements bound to the same protein factors in vitro. These proteins are the serum response factor (SRF) and the muscle actin promoter factors 1 and 2 (MAPF1 and MAPF2). The SRF and MAPF proteins were resolved by chromatographic procedures, and they differed in their relative affinities for each element. The factors were further distinguished by their distinct, but overlapping, methylation interference footprint patterns on each element. These data indicate that the differences in tissue-specific expression may be due to a complex interaction of protein factors with these sequences.

Amphibian (urodele) myotomes display transitory anterior/posterior and medial/lateral differentiation patterns

Myotome differentiation during Mexican axolotl (Ambystoma mexicanum) somitogenesis was analyzed by employing anti-actin and anti-myosin monoclonal antibodies as molecular probes. Myotome differentiation occurs after segmentation and proceeds in the cranial-to-caudal direction along the somite file. Within individual somites myotome differentiation displays distinct polarities. Examination of the somite file at the tailbud stage revealed that soon after segmentation, actin/myosin accumulate predominantly in the anterior and medial region of the myotome initially. Subsequently, cells within the myotome differentiate in an anterior-to-posterior and medial-to-lateral direction. Experimental analysis of presomitic paraxial mesoderm grafts before segmentation revealed that this transient myotome polarity is autonomous. Comparative analyses indicate that this myotome differentiation pattern is urodele specific. Cynops pyrrhogaster undergoes myotome differentiation like the axolotl, while two anurans, Xenopus laevis and Bombina orientalis, do not.

Intracellular localisation and expression of mammalian CDC2 protein during myogenic differentiation

Myogenic differentiation involves withdrawal of myoblasts from the cell cycle and fusion to form multinucleate myotubes. To examine the role that cell cycle control genes may play in this process, we investigated the steady state levels of CDC2 protein and RNA during myogenesis of L6E9 rat myoblasts. Indirect immunofluorescence using a CDC2 affinity-purified antibody showed that this protein is localised exclusively in the cytoplasm with a higher concentration perinuclearly. Both protein and RNA levels were down-regulated to similar extents early in the differentiation process, as cells became quiescent. There was a further down-regulation of protein after fusion to form myotubes. Autonomous expression of CDC2 protein in L6E9 cells, after stable transfection with a metallothionein: CDC2 gene construct, failed to inhibit the differentiation process. This suggests that, although there is down-regulation in levels of CDC2 RNA and protein during myogenesis, this phenomenon per se does not play a primary role in controlling the differentiation process. If CDC2 is involved in control of differentiation, this must depend on post-translational modification of the protein.

5' flanking and first intron sequences of the human beta-actin gene required for efficient promoter activity

We have identified a CCAAT box element that is required for the efficient transcription of the human beta-actin gene. Both in vivo transient transfection assays in cultured HeLa cells and in vitro run-off transcription assays in HeLa whole cell extracts demonstrated the requirement of this element for efficient promoter activity. A gel mobility shift assay revealed a Hela nuclear factor that specifically interacted with the beta-actin CCAAT element in vitro; mutation of the first three base pairs of the CCAAT pentanucleotide abolished binding of this factor. Competition gel shift experiments revealed that three sequence elements located within the beta-actin promoter, each containing a CC(A/T)6GG motif similar to that contained within the c-fos serum response element, were able to bind a different nuclear factor, serum response factor (SRF). One of these CC(A/T)6GG motifs is contained within a first intron fragment that enhanced transcription from a heterologous promoter in vivo.

Expression of the troponin complex genes: transcriptional coactivation during myoblast differentiation and independent control in heart and skeletal muscles

We compared the developmental regulation of the three troponin genes that encode the proteins of the Ca2+ regulatory complex in striated muscles of the Japanese quail. Nuclear run-on transcription and RNA protection analyses showed that the fast skeletal troponin I, the fast skeletal troponin T, and the slow skeletal-cardiac troponin C genes were transcriptionally coactivated and that transcripts rapidly accumulated within 6 to 12 h after the initiation of myoblast differentiation. The fast-isoform mRNAs of troponin I and troponin T were coexpressed at similar levels in different skeletal muscles, whereas the slow-cardiac troponin C mRNA varied independently and was the only one of these genes expressed in embryonic and adult heart. We conclude that these troponin genes are transcriptionally coactivated during skeletal myoblast differentiation, indicating that their transcription is under precise temporal control. However, this troponin C gene is regulated independently is specialized striated muscles.

Alpha-smooth muscle actin is transiently expressed in embryonic rat cardiac and skeletal muscles

Actin isoform expression may change during development, and in certain physiological, experimental and pathological situations. It is accepted that during sarcomeric (skeletal and cardiac) muscle development, the alpha-skeletal and alpha-cardiac isoforms of actin accumulate rapidly at the onset of muscle fibre formation, while there is a rapid fall in the expression of nonmuscle (beta and gamma) actin isoforms. Here we show that, before birth, both skeletal and myocardial cells express significant amounts of alpha-smooth muscle actin mRNA and protein. This expression is transient and disappears over the 1-7 days following birth. Our findings show that the program regulating actin isoform expression in sarcomeric muscle development is complex and that alpha-smooth muscle actin participates in this process.

Replicating myoblasts express a muscle-specific phenotype

During the terminal stage of skeletal myogenesis, myoblasts stop replicating, fuse to form multinucleate fibers, and express the genes that encode the proteins that convey contractile capacity. Because of this dramatic shift in proliferative state, morphology, and gene expression, it has been possible to readily identify and quantitate terminally differentiating myoblasts. In contrast, it is not clear whether the proliferating cells that give rise to postmitotic myoblasts are equally distinct in their phenotype and in fact whether distinct stages in skeletal myogenesis precede the onset of terminal differentiation. To address these questions, monoclonal antibodies and immunofluorescence microscopy were used to determine that replicating myoblasts from newborn rats do express a muscle-specific phenotype. To identify replicating cells, incorporation of 5-bromo-2'-deoxyuridine (BrdUrd) into DNA was assayed by using anti-BrdUrd antibody. The developmentally regulated, muscle-specific, integral membrane protein H36 and the intermediate-filament protein desmin were scored as markers of the myogenic phenotype. The percentage of BrdUrd+ (i.e., proliferative) cells among H36+ and desmin+ myoblasts was equal to the percentage of BrdUrd+ cells in the entire population, indicating that the expression of H36 and desmin is uniformly characteristic of replicating myoblasts. Inhibition of protein synthesis before and during growth in BrdUrd did not alter the frequency of desmin and H36 immunofluorescence in BrdUrd+ cells. Thus, desmin and H36 were present in the replicating myoblasts prior to the onset of growth in BrdUrd. These results were confirmed using H36+ cells selected by flow cytometry: these purified H36+ myoblasts replicate, express desmin, and differentiate. Similar results were obtained with mouse myoblasts. Desmin expression in these mammalian cells differs from that in chicken embryo myoblasts: only a small proportion of replicating chicken embryo myoblasts express desmin. That replicating mammalian myoblasts have a muscle-specific phenotype serves to define a distinct stage in myogenic development and a specific cell in the myogenic lineage. Further, it implies that there is a regulatory event activated during myogenesis that precedes terminal differentiation and that is required for expression of those genes whose products distinguish the replicating myoblast.

Fast skeletal muscle-specific expression of a quail troponin I gene in transgenic mice

We have produced seven lines of transgenic mice carrying the quail gene encoding the fast skeletal muscle-specific isoform of troponin I (TnIf). The quail DNA included the entire TnIf gene, 530 base pairs of 5'-flanking DNA, and 1.5 kilobase pairs of 3'-flanking DNA. In all seven transgenic lines, normally initiated and processed quail TnIf mRNA was expressed in skeletal muscle, where it accumulated to levels comparable to that in quail muscle. Moreover, in the three lines tested, quail TnIf mRNA levels were manyfold higher in a fast skeletal muscle (gastrocnemius) than in a slow skeletal muscle (soleus). We conclude that the cellular mechanisms directing muscle fiber type-specific TnIf gene expression are mediated by cis-regulatory elements present on the introduced quail DNA fragment and that they control TnIf expression by affecting the accumulation of TnIf mRNA. These elements have been functionally conserved since the evolutionary divergence of birds and mammals, despite the major physiological and morphological differences existing between avian (tonic) and mammalian (twitch) slow muscles. In lines of transgenic mice carrying multiple tandemly repeated copies of the transgene, an aberrant quail TnIf transcript (differing from normal TnIf mRNA upstream of exon 2) also accumulated in certain tissues, particularly lung, brain, spleen, and heart tissues. However, this aberrant transcript was not detected in a transgenic line which carries only a single copy of the quail gene.

Sequential activation of alpha-actin genes during avian cardiogenesis: vascular smooth muscle alpha-actin gene transcripts mark the onset of cardiomyocyte differentiation

The expression of cytoplasmic beta-actin and cardiac, skeletal, and smooth muscle alpha-actins during early avian cardiogenesis was analyzed by in situ hybridization with mRNA-specific single-stranded DNA probes. The cytoplasmic beta-actin gene was ubiquitously expressed in the early chicken embryo. In contrast, the alpha-actin genes were sequentially activated in avian cardiac tissue during the early stages of heart tube formation. The accumulation of large quantities of smooth muscle alpha-actin transcripts in epimyocardial cells preceded the expression of the sarcomeric alpha-actin genes. The accumulation of skeletal alpha-actin mRNAs in the developing heart lagged behind that of cardiac alpha-actin by several embryonic stages. At Hamburger-Hamilton stage 12, the smooth muscle alpha-actin gene was selectively down-regulated in the heart such that only the conus, which subsequently participates in the formation of the vascular trunks, continued to express this gene. This modulation in smooth muscle alpha-actin gene expression correlated with the beginning of coexpression of sarcomeric alpha-actin transcripts in the epimyocardium and the onset of circulation in the embryo. The specific expression of the vascular smooth muscle alpha-actin gene marks the onset of differentiation of cardiac cells and represents the first demonstration of coexpression of both smooth muscle and striated alpha-actin genes within myogenic cells.

Expression of the troponin complex genes: transcriptional coactivation during myoblast differentiation and independent control in heart and skeletal muscles

We compared the developmental regulation of the three troponin genes that encode the proteins of the Ca2+ regulatory complex in striated muscles of the Japanese quail. Nuclear run-on transcription and RNA protection analyses showed that the fast skeletal troponin I, the fast skeletal troponin T, and the slow skeletal-cardiac troponin C genes were transcriptionally coactivated and that transcripts rapidly accumulated within 6 to 12 h after the initiation of myoblast differentiation. The fast-isoform mRNAs of troponin I and troponin T were coexpressed at similar levels in different skeletal muscles, whereas the slow-cardiac troponin C mRNA varied independently and was the only one of these genes expressed in embryonic and adult heart. We conclude that these troponin genes are transcriptionally coactivated during skeletal myoblast differentiation, indicating that their transcription is under precise temporal control. However, this troponin C gene is regulated independently is specialized striated muscles.

Upstream regulatory region for inducible expression of the chicken skeletal myosin alkali light-chain gene

The expression of the fast type of myosin alkali light chain 1 is induced during the differentiation of muscle cells. To study the mechanism of its gene regulation, we joined the sequence of the 5'-flanking and upstream region of the chicken myosin alkali light-chain gene to the structural gene for chloramphenicol acetyltransferase (CAT). The fusion gene was introduced either into quail myoblasts transformed by a temperature-sensitive mutant of Rous sarcoma virus (tsNY68) or into chicken myoblasts, and the transiently expressed CAT activity was assayed after the differentiation of the myoblasts. From the experiments with the external and internal deletion mutants of the fusion gene, the cis-acting regulatory region responsible for the enhanced expression of the CAT activity in response to the cell differentiation was found to be localized at 2 kilobases upstream of the transcription initiation site. This region of 160 nucleotides contained two pairs of short sequences worthy of note, a direct repeat of 12 nucleotides, and an inverted repeat of 8 nucleotides. The nucleotide sequences of the 5'-flanking sequence up to nucleotide -3381 were determined and compared with those of the upstream activating elements of actin genes.

Isolation and characterization of a variant myoblast cell line that is temperature sensitive for differentiation

A new variant rat myogenic cell line, ts485, was isolated by subcloning the cell line ts3b2 (H. T. Nguyen, R. M. Medford, and B. Nadal-Ginard, Cell 34:281-293, 1983). Unlike the progenitor cell line, ts485 was thermosensitive for differentiation. Experiments with conditioned medium suggested that diffusible extracellular factors were not involved in dictating the differential phenotypes of ts485 cells cultured at the permissive and nonpermissive temperatures. Temperature shift experiments performed on cultures of ts485 cells indicated that the temperature-sensitive lesion was in a factor active during the growth phase and required to trigger a cascade of events leading to terminal differentiation.

Posttranscriptional control of embryonic rat skeletal muscle protein synthesis. Control at the level of translation by endogenous RNA

The onset of muscle cell differentiation is associated with increased transcription of muscle-specific mRNA. Studies from this laboratory using 19-d embryonic rat skeletal muscle, suggest that additional, posttranscriptional controls regulate maturation of muscle tissue via a quantitative effect upon translation, and that the regulatory component may reside within the poly A- RNA pool (Nathanson, M.A., E.W. Bush, and C. Vanderburg. 1986. J. Biol. Chem. 261:1477-1486). To further characterize muscle cell translational control, embryonic and adult total RNA were separated into oligo(dT)cellulose-bound (poly A+) and -unbound (poly A-) pools. Unbound material was subjected to agarose gel electrophoresis to resolve constituents of varying molecular size and mechanically cut into five fractions. Material of each fraction was electroeluted and recovered by precipitation. Equivalent loads of total RNA from 19-20-d embryonic rat skeletal muscle exhibited a 40% translational inhibition in comparison to its adult counterpart. Inhibition was not due to decreased message abundance because embryonic, as well as adult muscle, contained equivalent proportions of poly A+ mRNA. An inhibition assay, based upon the translatability of adult RNA and its inhibition by embryonic poly A- RNA, confirmed that inhibition was associated with a 160-2,000-nt poly A- fraction. Studies on the chemical composition of this fraction confirmed its RNA composition, the absence of ribonucleoprotein, and that its activity was absent from similarly fractionated adult RNA. Rescue of inhibition could be accomplished by addition of extra lysate or mRNA; however, smaller proportions of lysate were required, suggesting a strong interaction of inhibitor and components of the translational apparatus. Additional studies demonstrated that the inhibitor acted at the level of initiation, in a dose-dependent fashion. The present studies confirm the existence of translational control in skeletal muscle and suggest that it operates at the embryonic to adult transition. A model of muscle cell differentiation, based upon transcriptional control at the myoblast level, followed by translational regulation at the level of the postmitotic myoblast and/or myotube, is proposed.

Increased levels of acetylcholine receptor alpha-subunit mRNA in experimental autoimmune myasthenia gravis

To gain insight into the regulatory mechanisms underlying the blockade and loss of acetylcholine receptor (AChR) in myasthenia, we have followed AChR alpha-subunit mRNA levels in leg muscles of myasthenic and normal rabbits and rats. Northern blots of RNA preparations from normal and myasthenic animals were hybridized with a mouse AChR alpha-subunit cDNA probe. Our experiments indicate a specific increase (4-7-fold) in the levels of alpha-subunit mRNA in animals with experimental autoimmune myasthenia gravis (EAMG), in comparison with control animals. Actin mRNA levels were essentially unchanged. Our results thus suggest that EAMG is accompanied by an increased level of AChR gene transcription.

Isolation and characterization of a variant myoblast cell line that is temperature sensitive for differentiation

A new variant rat myogenic cell line, ts485, was isolated by subcloning the cell line ts3b2 (H. T. Nguyen, R. M. Medford, and B. Nadal-Ginard, Cell 34:281-293, 1983). Unlike the progenitor cell line, ts485 was thermosensitive for differentiation. Experiments with conditioned medium suggested that diffusible extracellular factors were not involved in dictating the differential phenotypes of ts485 cells cultured at the permissive and nonpermissive temperatures. Temperature shift experiments performed on cultures of ts485 cells indicated that the temperature-sensitive lesion was in a factor active during the growth phase and required to trigger a cascade of events leading to terminal differentiation.

Upstream regulatory region for inducible expression of the chicken skeletal myosin alkali light-chain gene

The expression of the fast type of myosin alkali light chain 1 is induced during the differentiation of muscle cells. To study the mechanism of its gene regulation, we joined the sequence of the 5'-flanking and upstream region of the chicken myosin alkali light-chain gene to the structural gene for chloramphenicol acetyltransferase (CAT). The fusion gene was introduced either into quail myoblasts transformed by a temperature-sensitive mutant of Rous sarcoma virus (tsNY68) or into chicken myoblasts, and the transiently expressed CAT activity was assayed after the differentiation of the myoblasts. From the experiments with the external and internal deletion mutants of the fusion gene, the cis-acting regulatory region responsible for the enhanced expression of the CAT activity in response to the cell differentiation was found to be localized at 2 kilobases upstream of the transcription initiation site. This region of 160 nucleotides contained two pairs of short sequences worthy of note, a direct repeat of 12 nucleotides, and an inverted repeat of 8 nucleotides. The nucleotide sequences of the 5'-flanking sequence up to nucleotide -3381 were determined and compared with those of the upstream activating elements of actin genes.

Control of myogenesis in the mouse myogenic C2 cell line by medium composition and by insulin: characterization of permissive and inducible C2 myoblasts

Using subcloning and manipulations of culture conditions we have isolated from the mouse myogenic cell line C2 a variant cell line that we named inducible. Unlike the progenitor cells that are referred to as permissive, inducible myoblasts differentiate poorly in Dulbecco modified Eagle medium plus fetal calf serum (FCS) and require the presence of insulin at a high concentration (1.6 10(-6) M) or insulin-like growth factor I (IGFI) at a lower concentration (2.5 10(-8) M) to differentiate. Permissive and inducible myoblasts fail to differentiate when grown in MCDB202 medium plus 20% FCS, even after a prolonged arrest in G1 phase. This shows that an arrest in G1 is in itself insufficient to trigger terminal differentiation. Both cell types also exhibit distinct patterns of accumulation of muscle mRNAs corresponding to sarcomeric actins and myosin light chain MLC1A. The possibility that these two cell lines might represent two different stages of the progression of myoblasts toward terminal differentiation is discussed.