Cellular localization of muscle and nonmuscle actin mRNAs in chicken primary myogenic cultures: the induction of alpha-skeletal actin mRNA is regulated independently of alpha-cardiac actin gene expression

Monoclonal antibodies against muscle actin isoforms: epitope identification and analysis of isoform expression by immunoblot and immunostaining in normal and regenerating skeletal muscle

Higher vertebrates (mammals and birds) express six different highly conserved actin isoforms that can be classified in three subgroups: 1) sarcomeric actins, α-skeletal (α-SKA) and α-cardiac (α-CAA), 2) smooth muscle actins (SMAs), α-SMA and γ-SMA, and 3) cytoplasmic actins (CYAs), β-CYA and γ-CYA. The variations among isoactins, in each subgroup, are due to 3-4 amino acid differences located in their acetylated N-decapeptide sequence. The first monoclonal antibody (mAb) against an actin isoform (α-SMA) was produced and characterized in our laboratory in 1986 (Skalli  et al., 1986) . We have further obtained mAbs against the 5 other isoforms. In this report, we focus on the mAbs anti-α-SKA and anti-α-CAA obtained after immunization of mice with the respective acetylated N-terminal decapeptides using the Repetitive Immunizations at Multiple Sites Strategy (RIMMS). In addition to the identification of their epitope by immunoblotting, we describe the expression of the 2 sarcomeric actins in mature skeletal muscle and during muscle repair after micro-lesions. In particular, we analyze the expression of α-CAA, α-SKA and α-SMA by co-immunostaining in a time course frame during the muscle repair process. Our results indicate that a restricted myocyte population expresses α-CAA and suggest a high capacity of self-regeneration in muscle cells. These antibodies may represent a helpful tool for the follow-up of muscle regeneration and pathological changes.

Involvement of unconventional myosin VI in myoblast function and myotube formation

The important role of unconventional myosin VI (MVI) in skeletal and cardiac muscle has been recently postulated (Karolczak et al. in Histochem Cell Biol 139:873-885, 2013). Here, we addressed for the first time a role for this unique myosin motor in myogenic cells as well as during their differentiation into myotubes. During myoblast differentiation, the isoform expression pattern of MVI and its subcellular localization underwent changes. In undifferentiated myoblasts, MVI-stained puncti were seen throughout the cytoplasm and were in close proximity to actin filaments, Golgi apparatus, vinculin-, and talin-rich focal adhesion as well as endoplasmic reticulum. Colocalization of MVI with endoplasmic reticulum was enhanced during myotube formation, and differentiation-dependent association was also seen in sarcoplasmic reticulum of neonatal rat cardiomyocytes (NRCs). Moreover, we observed enrichment of MVI in myotube regions containing acetylcholine receptor-rich clusters, suggesting its involvement in the organization of the muscle postsynaptic machinery. Overexpression of the H246R MVI mutant (associated with hypertrophic cardiomyopathy) in myoblasts and NRCs caused the formation of abnormally large intracellular vesicles. MVI knockdown caused changes in myoblast morphology and inhibition of their migration. On the subcellular level, MVI-depleted myoblasts exhibited aberrations in the organization of actin cytoskeleton and adhesive structures as well as in integrity of Golgi apparatus and endoplasmic reticulum. Also, MVI depletion or overexpression of H246R mutant caused the formation of significantly wider or aberrant myotubes, respectively, indicative of involvement of MVI in myoblast differentiation. The presented results suggest an important role for MVI in myogenic cells and possibly in myoblast differentiation.

Rotenone inhibits primary murine myotube formation via Raf-1 and ROCK2

Rotenone (ROT) is a widely used inhibitor of complex I (CI), the first complex of the mitochondrial oxidative phosphorylation (OXPHOS) system. However, particularly at high concentrations ROT was also described to display off-target effects. Here we studied how ROT affected in vitro primary murine myotube formation. We demonstrate that myotube formation is specifically inhibited by ROT (10-100nM), but not by piericidin A (PA; 100nM), another CI inhibitor. At 100nM, both ROT and PA fully blocked myoblast oxygen consumption. Knock-down of Rho-associated, coiled-coil containing protein kinase 2 (ROCK2) and, to a lesser extent ROCK1, prevented the ROT-induced inhibition of myotube formation. Moreover, the latter was reversed by inhibiting Raf-1 activity. In contrast, ROT-induced inhibition of myotube formation was not prevented by knock-down of RhoA. Taken together, our results support a model in which ROT reduces primary myotube formation independent of its inhibitory effect on CI-driven mitochondrial ATP production, but via a mechanism primarily involving the Raf-1/ROCK2 pathway.

Complex interactions between human myoblasts and the surrounding 3D fibrin-based matrix

Anchorage of muscle cells to the extracellular matrix is crucial for a range of fundamental biological processes including migration, survival and differentiation. Three-dimensional (3D) culture has been proposed to provide a more physiological in vitro model of muscle growth and differentiation than routine 2D cultures. However, muscle cell adhesion and cell-matrix interplay of engineered muscle tissue remain to be determined. We have characterized cell-matrix interactions in 3D muscle culture and analyzed their consequences on cell differentiation. Human myoblasts were embedded in a fibrin matrix cast between two posts, cultured until confluence, and then induced to differentiate. Myoblasts in 3D aligned along the longitudinal axis of the gel. They displayed actin stress fibers evenly distributed around the nucleus and a cortical mesh of thin actin filaments. Adhesion sites in 3D were smaller in size than in rigid 2D culture but expression of adhesion site proteins, including α5 integrin and vinculin, was higher in 3D compared with 2D (p<0.05). Myoblasts and myotubes in 3D exhibited thicker and ellipsoid nuclei instead of the thin disk-like shape of the nuclei in 2D (p<0.001). Differentiation kinetics were faster in 3D as demonstrated by higher mRNA concentrations of α-actinin and myosin. More important, the elastic modulus of engineered muscle tissues increased significantly from 3.5 ± 0.8 to 7.4 ± 4.7 kPa during proliferation (p<0.05) and reached 12.2 ± 6.0 kPa during differentiation (p<0.05), thus attesting the increase of matrix stiffness during proliferation and differentiation of the myocytes. In conclusion, we reported modulations of the adhesion complexes, the actin cytoskeleton and nuclear shape in 3D compared with routine 2D muscle culture. These findings point to complex interactions between muscle cells and the surrounding matrix with dynamic regulation of the cell-matrix stiffness.

Regulation of motility of myogenic cells in filling limb muscle anlagen by Pitx2

Cells of the ventrolateral dermomyotome delaminate and migrate into the limb buds where they give rise to all muscles of the limbs. The migratory cells proliferate and form myoblasts, which withdraw from the cell cycle to become terminally differentiated myocytes. The myogenic lineage colonizes pre-patterned regions to form muscle anlagen as muscle fibers are assembled. The regulatory mechanisms that control the later steps of this myogenic program are not well understood. The homeodomain transcription factor Pitx2 is expressed specifically in the muscle lineage from the migration of precursors to adult muscle. Ablation of Pitx2 results in distortion, rather than loss, of limb muscle anlagen, suggesting that its function becomes critical during the colonization of, and/or fiber assembly in, the anlagen. Microarrays were used to identify changes in gene expression in flow-sorted migratory muscle precursors, labeled by Lbx1(EGFP/+), which resulted from the loss of Pitx2. Very few genes showed changes in expression. Many small-fold, yet significant, changes were observed in genes encoding cytoskeletal and adhesion proteins which play a role in cell motility. Myogenic cells from genetically-tagged mice were cultured and subjected to live cell-tracking analysis using time-lapse imaging. Myogenic cells lacking Pitx2 were smaller, more symmetrical, and had more actin bundling. They also migrated about half of the total distance and velocity. Decreased motility may prevent myogenic cells from filling pre-patterned regions of the limb bud in a timely manner. Altered shape may prevent proper assembly of higher-order fibers within anlagen. Pitx2 therefore appears to regulate muscle anlagen development by appropriately balancing expression of cytoskeletal and adhesion molecules.

Local tissue geometry determines contractile force generation of engineered muscle networks

The field of skeletal muscle tissue engineering is currently hampered by the lack of methods to form large muscle constructs composed of dense, aligned, and mature myofibers and limited understanding of structure-function relationships in developing muscle tissues. In our previous studies, engineered muscle sheets with elliptical pores ("muscle networks") were fabricated by casting cells and fibrin gel inside elastomeric tissue molds with staggered hexagonal posts. In these networks, alignment of cells around the elliptical pores followed the local distribution of tissue strains that were generated by cell-mediated compaction of fibrin gel against the hexagonal posts. The goal of this study was to assess how systematic variations in pore elongation affect the morphology and contractile function of muscle networks. We found that in muscle networks with more elongated pores the force production of individual myofibers was not altered, but the myofiber alignment and efficiency of myofiber formation were significantly increased yielding an increase in the total contractile force despite a decrease in the total tissue volume. Beyond a certain pore length, increase in generated contractile force was mainly contributed by more efficient myofiber formation rather than enhanced myofiber alignment. Collectively, these studies show that changes in local tissue geometry can exert both direct structural and indirect myogenic effects on the functional output of engineered muscle. Different hydrogel formulations and pore geometries will be explored in the future to further augment contractile function of engineered muscle networks and promote their use for basic structure-function studies in vitro and, eventually, for efficient muscle repair in vivo.

Actin isoform expression patterns during mammalian development and in pathology: insights from mouse models

The dynamic actin cytoskeleton, consisting of six actin isoforms in mammals and a variety of actin binding proteins is essential for all developmental processes and for the viability of the adult organism. Actin isoform specific functions have been proposed for muscle contraction, cell migration, endo- and exocytosis and maintaining cell shape. However, these specific functions for each of the actin isoforms during development are not well understood. Based on transgenic mouse models, we will discuss the expression patterns of the six conventional actin isoforms in mammals during development and adult life. Ablation of actin genes usually leads to lethality and affects expression of other actin isoforms at the cell or tissue level. A good knowledge of their expression and functions will contribute to fully understand severe phenotypes or diseases caused by mutations in actin isoforms.

Cytoskeleton/stretch-activated ion channel interaction regulates myogenic differentiation of skeletal myoblasts

In the present study, we investigated the functional interaction between stress fibers (SFs) and stretch-activated channels (SACs) and its possible role in the regulation of myoblast differentiation induced by switch to differentiation culture in the presence or absence of sphingosine 1-phosphate. It was found that there was a clear temporal correlation between SF formation and SAC activation in differentiating C2C12 myoblasts. Inhibition of actin polymerization with the specific Rho kinase inhibitor Y-27632, significantly decreased SAC sensitivity in these cells, suggesting a role for Rho-dependent actin remodeling in the regulation of the channel opening. The alteration of cytoskeletal/SAC functional correlation had also deleterious effects on myogenic differentiation of C2C12 cells as judged by combined confocal immunofluorescence, biochemical and electrophysiological analyses. Indeed, the treatment with Y-27632 or with DHCB, an actin disrupting agent, inhibited the expression of the myogenic markers (myogenin and sarcomeric proteins) and myoblast-myotube transition. The treatment with the channel blocker, GdCl(3), also affected myogenesis in these cells. It impaired, in fact, myoblast phenotypic maturation (i.e., reduced the expression of alpha-sarcomeric actin and skeletal myosin and the activity of creatine kinase) but did not modify promoter activity and protein expression levels of myogenin. The results of this study, together with being in agreement with the general idea that cytoskeletal remodeling is essential for muscle differentiation, describe a novel pathway whereby the formation of SFs and their contraction, generate a mechanical tension to the plasma membrane, activate SACs and trigger Ca(2+)-dependent signals, thus influencing the phenotypic maturation of myoblasts.

The cardiac isoform of α-actin in regenerating and atrophic skeletal muscle, myopathies and rhabdomyomatous tumors: an immunohistochemical study using monoclonal antibodies

The two sarcomeric isoforms of actins, cardiac and skeletal muscle alpha-actin, are highly homologous so that their immunohistochemical distinction is extremely difficult. Taking advantage of monoclonal antibodies distinguishing the two conservative amino acid exchanges near the aminoterminus, we have performed an extended immunohistochemical analysis of the cardiac alpha-actin (CAA) isoform in normal, regenerating, diseased and neoplastic human muscle tissues. Intense and uniform CAA staining is seen in fetal and adult myocardium and in fetal skeletal muscle while adult skeletal muscle is essentially negative, except for muscle spindle myocytes and a few scattered muscle fibres with overall reduced diameter. By contrast, CAA synthesis is markedly induced in regenerating skeletal muscle cells, in Duchenne muscular dystrophy and upon degenerative atrophy. CAA has also been detected in certain vascular and visceral smooth muscle cells. Among tumors, CAA has consistently been seen in rhabdomyosarcomas and rhabdomyomatous cells of nephroblastomas, whereas, smooth muscle tumors have shown only occasional staining. While the synthesis of this actin isoform is less restricted than previously thought, monoclonal antibodies against CAA provide a well-defined, reliable and sensitive diagnostic tool for the definition and detection of aberrant differentiation in diseased skeletal muscle and of striated muscle differentiation in rhabdomyosarcomas.

Functional specificity of actin isoforms

Actin, one of the main proteins of muscle and cytoskeleton, exists as a variety of highly conserved isoforms whose distribution in vertebrates is tissue-specific. Synthesis of specific actin isoforms is accompanied by their subcellular compartmentalization, with both processes being regulated by factors of cell proliferation and differentiation. Actin isoforms cannot substitute for each other, and the high-level synthesis of exogenous actins leads to alterations in cell organization and morphology. This indicates that the highly conserved actins are functionally specialized for the tissues in which they predominate. The first goal of this review is to analyze the data on the polymerizability of actin isoforms to show that cytoskeleton isoactins form less stable polymers than skeletal muscle actin. This difference correlates with the dynamics of actin microfilaments versus the stability of myofibrillar systems. The three-dimensional actin structure as well as progress in the analysis of conformational changes in both the actin monomer and the filament allows us to view the data on the structure and polymerization of isoactins in terms of structure-function relationships within the actin molecule. Most of the amino acid substitutions that distinguish actin isoforms are located apart from actin-actin contact sites in the polymer. We suggest that these substitutions can modulate the ability of actin monomers to form more or less stable polymers by long-range (allosteric) regulation of the contact sites.

Trypanosoma cruzi infection affects actin mRNA regulation in heart muscle cells

We have previously described alterations in the cytoskeletal organization of heart muscle cells (HMC) infected with Trypanosoma cruzi in vitro. Our aim was to investigate whether these changes also affect the regulation of the actin mRNAs during HMC differentiation. Northern blot analysis revealed that alpha-cardiac actin mRNA levels increased during cell differentiation while beta-actin mRNA levels declined. Nonmuscle cells displayed beta-actin mRNA signal localized at the cell periphery, while alpha-cardiac actin mRNA had a perinuclear distribution in myocytes. Trypanosoma cruzi-infected cells showed 50% reduction in alpha-cardiac actin mRNA expression after 72 h of infection. In contrast, beta-actin mRNA levels increased approximately 79% after 48 h of infection. In addition, in situ beta-actin mRNA was delocalized from the periphery into the perinuclear region. These observations support the hypothesis that Trypanosoma cruzi affects actin mRNA regulation and localization through its effect on the cytoskeleton of heart muscle cells.

Skeletal and cardiac alpha-actin isoforms exhibit unanticipated temporal and tissue-specific gene expression patterns in developing avian limbs and embryos

The initial expression of skeletal muscle structural genes typically occurs after myogenic determination factor gene expression. We investigated this temporal relationship via a reverse transcription-polymerase chain reaction (RT-PCR) analysis of skeletal and cardiac alpha-actin (s- and c-actin) mRNA during chick limb development. c-actin transcripts were first detected at the beginning of muscle cytodifferentiation in stage 24/25 limbs, shortly after the initial appearance of MyoD and myogenin mRNAs, and were not detected in nonmyogenic tissues. In contrast, s-actin mRNA was detected in limbs at stage 15-16, periods when myf5 and MRF4 but not MyoD and myogenin transcripts are present (Lin-Jones, J., and Hauschka, S. D., Dev. Biol. 174, 407-422, 1996). While s-actin mRNA was not detected in E7 neural retina and was at variable levels in stage 9-15 neural tube, significant levels were consistently detected in mesodermal tissues which contribute nonmyogenic cells to the limb: stage 9-12 lateral plate and distal portions of stage 25/26 limbs. s-actin transcripts detected in the earliest limbs could thus be in myogenic and/or nonmyogenic cells. These data indicate that while c-actin expression is activated at the onset of limb muscle cytodifferentiation, s-actin expression occurs much earlier, as well as in some nonmyogenic tissues. Whether the precocious expression of s-actin plays a functional role in muscle development remains to be determined.

Tissue and developmental specific expression of murine smooth muscle gamma-actin fusion genes in transgenic mice

Smooth muscle gamma-actin (SMGA) is an excellent marker of smooth muscle differentiation because it is essentially restricted to smooth muscle. As a first step toward unraveling the mechanisms underlying smooth muscle development and differentiation, we have examined the tissue-specific and developmental expression patterns of six constructs carrying portions of the murine SMGA gene linked to chloramphenicol acetyltransferase (CAT) in stable lines of transgenic mice. Based on the transgenic studies most, if not all, of the regulatory elements necessary for proper spatial and temporal expression of SMGA are present within a 13.7 kb segment of the SMGA gene containing 4.9 kb of upstream sequence, exon 1, intron 1, and a portion of exon 2 up to the start codon for translation. A second construct (SMGA11.6CAT) that lacks the distal 2.1 kb of upstream sequence but is otherwise identical to SMGA13.7CAT shows a similar level of smooth muscle-specific CAT activity. However, SMGA9.3CAT fusion gene containing only 571 bp of 5' flanking sequence, but otherwise identical to SMGA13.7CAT, and SMGA6.0CAT containing only the 4.9 kb upstream sequence, exon 1, and a miniintron 1 show a more than a 100-fold reduction of CAT activity in most smooth muscle-rich tissues. Furthermore, removal of most or all of intron 1 from a transgene with 571 bp of upstream sequence (SMGA2.0 CAT and SMGA0.6CAT) results in a near-complete or complete loss of activity, respectively, in all tissues. Overall, the studies suggest that upstream elements between -2.7 kb and -571 bp and elements within intron 1 are required for high levels of SMGA gene expression in an appropriate temporal-spatial fashion.

Specific immunohistochemical detection of cardiac/fetal alpha-actin in human cardiomyocytes and regenerating skeletal muscle cells

We describe three murine monoclonal antibodies (mAbs) raised against a synthetic decapeptide representing the aminoterminal sequence of the cardiac/ fetal isoform of sarcomeric alpha-actin. When used for immunoblotting or histological immunolocalization, these mAbs distinguish cardiac/fetal alpha-actin from skeletal muscle alpha-actin, and also from all other actin isoforms. We show, by immunofluorescence and immunoperoxidase microscopy of tissue sections, that cardiac/fetal alpha-actin can be localized not only in cardiomyocytes but also in skeletal muscles and their satellite cells during regeneration. These mAbs are potentially valuable in developmental biology, for the characterization of tissue and cultured myogenic cells, in pathology, and for serodiagnosis.

Pleomorphic soft tissue myogenic sarcomas of adulthood. A reappraisal in the mid-1990s

325 diverse sarcomas, 39 rhabdomyosarcomas (RMS), including all histologic variants, and 135 leiomyosarcomas (LMS) were identified. Within these two groups, 18 (46%) of the RMS and 14 (10%) of the LMS represented pleomorphic variants. These neoplasms were studied by morphology (histology and ultrastructure) and by immunohistochemical methods employing antibodies to intermediate filaments (vimentin and desmin) and actin isoforms [alpha-smooth (sm) and alpha-sarcomeric (sr) actins]. Twenty-four pleomorphic malignant fibrous histiocytomas (MFH) and eight pleomorphic liposarcomas (LS) were examined in a similar fashion. By light microscopy, the pleomorphic RMS, LMS, and MFH were indistinguishable, as each was dominated by pleomorphic cells disposed in a haphazard growth pattern; moreover, many featured fascicular, storiform, and sclerotic zones. The distinction between these neoplasms became apparent only following immunohistochemistry and/or ultrastructural study. All pleomorphic RMS disclosed rudimentary sarcomeres and exhibited the following cytoskeletal profile: vimentin (+) (18 of 18), desmin (+) (14 of 18), alpha-sr actin (+) (18 of 18) and alpha-sm actin (+) (five of 18). All the pleomorphic LMS featured smooth-muscle differentiation of variable degrees in the form of cytoplasmic bundles of microfilaments and associated dense bodies; their cytoskeletal profile was vimentin (+) (14 of 14), desmin (+) (seven of 14), alpha-sr actin (+) (none of 14), and alpha-sm actin (+) (eight of 14). The latter was demonstrated in all moderately differentiated, but absent or only focally expressed in poorly differentiated variants. All pleomorphic MFH and LS were devoid of myogenic (skeletal or smooth) ultrastructural features and expressed vimentin solely. This combined morphological and immunohistochemical study illustrates the following: First, these pleomorphic sarcomas are often indistinguishable by histologic growth pattern alone; thus, an accurate diagnosis requires study with all of these techniques. Second, pleomorphic myogenic sarcomas are restricted to adults and are not uncommon neoplasms among pleomorphic sarcomas: RMS (28%), LMS (21%), MFH (38%), and LS (13%). Third, the study defines desmin-negative and alpha-sm actin-positive pleomorphic RMS, and desmin-negative and alpha-sm-actin-negative pleomorphic LMS.

Heterogeneity in the progeny of single human muscle satellite cells

We examined whether freshly isolated (native) human muscle satellite cells (HMSC), as well as their proliferating clonal progenies, were heterogeneous. We studied the expression of the cytoskeletal proteins, desmin (DSM), alpha-sarcomeric and alpha-smooth muscle actins (alpha-SR actin, alpha-SM actin), three markers that may be expressed prior to the fusion process. We found that native HMSC constituted a homogeneous population of cells expressing desmin and giving rise to similar clones in vitro. The clonal progeny of HMSC was heterogeneous, including several subpopulations of myoblasts with different cytoskeletal phenotypes, commitment states and fusion abilities. A major subpopulation that expressed both alpha-sarcomeric actin and desmin during the proliferative stage corresponded to a "predifferentiated" population of myoblasts, committed to fusion. Another subpopulation, expressing exclusively desmin, and phenotypically similar to native HMSC, failed to fuse under fusion-promoting conditions and could represent a new generation of HMSC born in culture.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.

All histological types of primary human rhabdomyosarcoma express alpha-cardiac and not alpha-skeletal actin messenger RNA

Eleven human primary rhabdomyosarcomas (RMSs), including all histological variants, were analyzed morphologically, immunohistochemically for intermediate filament proteins and actin isoforms, and by means of Northern blots with probes specific for total actin, alpha-skeletal (SK), alpha-cardiac (CARD), and alpha-smooth muscle actin messenger (m)RNAs. All tumors disclosed ultrastructural evidence of skeletal muscle features with terminal differentiation in three cases. The RMSs contained immunohistochemically the intermediate filament proteins vimentin and desmin and reacted positively with the alpha-sarcomeric actin antibody, which recognizes alpha-SK and alpha-CARD actin isoforms. All RMSs reacted with the total actin probe, recognizing at 2.1 kb cytoplasmic actin mRNAs and at 1.7 kb alpha-actin mRNAs. With the specific probes, all RMSs expressed alpha-CARD actin mRNA, four neoplasms expressed also alpha-smooth muscle actin mRNA, whereas the probe for alpha-SK actin mRNA never produced a signal except in one case, in which the tumor masses were intermingled with non-neoplastic preexistent striated muscle fibers. Because alpha-CARD and alpha-smooth muscle actins are transiently expressed during normal skeletal muscle development, RMSs seem to follow normal skeletal myogenesis without completing the final step, consisting of alpha-SK actin mRNA expression. The use of Northern blots for alpha-CARD actin as an adjunct to conventional techniques may be helpful for the precise identification of primary RMSs compared to other soft tissue neoplasms.

Aphidicolin-resistant polyomavirus and subgenomic cellular DNA synthesis occur early in the differentiation of cultured myoblasts to myotubes

Small DNA viruses have been historically used as probes of cellular control mechanisms of DNA replication, gene expression, and differentiation. Polyomavirus (Py) DNA replication is known to be linked to differentiation of may cells, including myoblasts. In this report, we use this linkage in myoblasts to simultaneously examine (i) cellular differentiation control of Py DNA replication and (ii) an unusual type of cellular and Py DNA synthesis during differentiation. Early proposals that DNA synthesis was involved in the induced differentiation of myoblasts to myotubes were apparently disproved by reliance on inhibitors of DNA synthesis (cytosine arabinoside and aphidicolin), which indicated that mitosis and DNA replication are not necessary for differentiation. Theoretical problems with the accessibility of inactive chromatin to trans-acting factors led us to reexamine possible involvement of DNA replication in myoblast differentiation. We show here that Py undergoes novel aphidicolin-resistant net DNA synthesis under specific conditions early in induced differentiation of myoblasts (following delayed aphidicolin addition). Under similar conditions, we also examined uninfected myoblast DNA synthesis, and we show that soon after differentiation induction, a period of aphidicolin-resistant cellular DNA synthesis can also be observed. This drug-resistant DNA synthesis appears to be subgenomic, not contributing to mitosis, and more representative of polyadenylated than of nonpolyadenylated RNA. These results renew the possibility that DNA synthesis plays a role in myoblast differentiation and suggest that the linkage of Py DNA synthesis to differentiation may involve a qualitative cellular alteration in Py DNA replication.

Displacement of BrdUrd-induced YY1 by serum response factor activates skeletal alpha-actin transcription in embryonic myoblasts

Muscle-restricted transcription of the skeletal alpha-actin gene is controlled in part by a positive regulator, serum response factor (SRF), and a negative regulator, F-ACT1, which bind competitively to the most proximal serum response element (SRE1). We show here that F-ACT1 is identical to a transcription factor recently cloned and described as YY1, NF-E1, delta, or UCRBP. We found that although the DNA-binding activity of SRF accumulates during myogenesis, that of YY1 diminishes simultaneously. Myoblasts rendered incapable of differentiation by BrdUrd treatment exhibited the highest level of YY1 and the lowest level of SRF activities. Transfected SRF could directly transactivate the skeletal alpha-actin promoter by overcoming the inhibitory effect of BrdUrd-induced YY1. The transactivation depends on intact SRE DNA elements and requires the DNA-binding/dimerization domain of SRF as well as its C-terminal half rich in serines and threonines. Since the functions of YY1 and SRF appear to be developmentally regulated, the convergence of their binding sites upon the SRE constitutes an integrated mechanism whereby temporal and spatial muscle gene expression may be accomplished.

Onset of expression and regional deposition of alpha-smooth and sarcomeric actin during avian heart development

The sequential appearance of mRNAs for smooth, cardiac, and skeletal alpha-actin has been described during development of the chicken heart (Ruzicka, D.L., and R.J. Schwartz 1988 J. Cell Biol., 107:2575-2586). To assess whether this reflects the deposition of corresponding isoproteins, we have immunocytochemically localized smooth and sarcomeric (cardiac and skeletal) alpha-actin in Hamburger-Hamilton (H-H) stage 7-18 embryos using monoclonal antibodies. Within the developing embryo at stage 9-, smooth muscle alpha-actin was exclusively detected in the developing heart, upon fusion of the endocardial tubes; sarcomeric alpha-actin was observed later (stage 9). By the onset of contraction at stage 10+, intense immunostaining of both smooth and sarcomeric isoproteins was observed in the ventricle; at this time smooth muscle alpha-actin was also detected in splanchnic mesoderm of the pre-vitelline area, in a cellular layer adjacent to the only embryonic cells that exhibited factor VIII (von Willebrand factor) antigens. Double immunostaining of the myocardium at stage 11, at which time striations were first detected, revealed the co-existence of smooth and sarcomeric actin in developing sarcomeres. Intense expression of sarcomeric actin continued in the heart after stage 11, whereas smooth muscle alpha-actin was down-regulated in the ventricle and became regionalized to the inflow and outflow tracts. As expected, smooth muscle alpha-actin was detected around intra- and extra-embryonic vascular structures at later developmental stages, while sarcomeric actin was observed in somites.

Mechanism of tissue-specific transcription: interplay between positive and negative regulatory factors

At least four regulatory cis-acting DNA sequences, CCAAAAGTGG (element A), TTATTTTTA (element B), TATTTATT (element C), and TATTACCTTTAT (element S), were identified in cardiac myosin light chain-2 (MLC2) proximal promoter as target sites for sequence-specific binding of nuclear proteins. For muscle-specific transcription, the proximal promoter (-53 to +1) consisting only of elements B and C is required. Addition of element A to this promoter results in a muscle-specific up-regulation, whereas the addition of element S exerts a negative effect on transcription. The negative and positive regulatory effects of elements S and A respectively were demonstrated by site-specific mutations of the promoter following transient transfection of cardiac muscle cells in culture. Elements S and A interact separately with distinct nuclear protein factor present in both muscle and non-muscle cells, even though their regulatory activities are restricted to muscle cells. Among the multiple complexes resulting from the interaction of nuclear proteins and elements S and A DNAs, one requires both S and A sequences together for binding. Element B, which exerts a muscle-specific positive effect on transcription, binds to a nuclear protein present in cardiac muscle, but not in non-muscle cells. DNA-protein binding assays and mutational analysis of the MLC2 promoter suggest that the contribution of the functionally opposed cis-elements depends upon an interplay between the positively and negatively acting DNA-binding proteins via protein-protein interactions to mediate opposite regulatory effects on gene transcription.

Phorbol esters selectively downregulate contractile protein gene expression in terminally differentiated myotubes through transcriptional repression and message destabilization

Chronic exposure of differentiated avian skeletal muscle cells in culture to the phorbol ester, 12-O-tetradecanoyl phorbol-13-acetate (PMA), results in the selective disassembly of sarcomeric structures and loss of muscle-specific contractile proteins, leaving cytoskeletal structures and their associated proteins intact. We demonstrate here that these morphological and biochemical changes are accompanied by dramatic and selective decreases in the level of the mRNAs that encode the contractile proteins. We measured the effects of PMA on the transcriptional activity and mRNA stability of four contractile protein genes (alpha-cardiac and alpha-skeletal actin, cardiac troponin C [cTnC], and myosin light chain lf [MLClf]) and two nonmuscle genes (beta-cytoplasmic actin and the glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase [GAPDH]). The transcriptional activity of the alpha-cardiac actin and cTnC genes dramatically decreased by 8 h after the addition of PMA, while other muscle and nonmuscle genes examined showed no change. Pulse-chase experiments of in vivo labeled RNA showed significant reductions in mRNA half-lifes for all the contractile protein mRNAs examined, while the half-lifes of beta-actin and GAPDH mRNA were unchanged. All of the above effects occurred under conditions in which cellular protein kinase C (PKC) levels had been reduced by greater than 90%. The fact that many of the contractile protein genes remained transcriptionally active despite the fact that the cells were unable to accumulate their mRNAs to any significant extent indicated that the treated cells were still committed to skeletal muscle differentiation. The selective changes in the stability of the contractile protein mRNAs suggest that the control of mRNA stability may be part of the normal regulatory program of skeletal muscle differentiation and that this control may be linked to the integrity of the contractile apparatus and mediated by second messenger pathways involving PKC activation.

Activation of skeletal alpha-actin gene transcription: the cooperative formation of serum response factor-binding complexes over positive cis-acting promoter serum response elements displaces a negative-acting nuclear factor enriched in replicating myoblasts and nonmyogenic cells

Three upstream CBAR cis-acting promoter elements, containing the inner core CC(A/T)6GG of the serum response element (SRE), are required for myogenic cell type-restricted expression of the avian skeletal alpha-actin gene (K.L. Chow and R.J. Schwartz, Mol. Cell. Biol. 10:528-538, 1990). These actin SRE elements display differential binding properties with two distinct nuclear proteins, serum response factor (SRF) and another factor described here as F-ACT1. SRF is able to bind to all actin SREs with various affinities. This multisite interaction is marked by cooperative binding events in that the two high-affinity proximal and distal SREs facilitate the weak central-site interaction with SRF, leading to the formation of a higher-order SRF-promoter complex. Functional analyses reveal that undisrupted multiple SRF-DNA interactions are absolutely essential for promoter activity in myogenic cells. F-ACT1, present at higher levels in nonmyogenic cells and replicating myoblasts than in myotubes, binds solely to the proximal SRE, and its binding is mutually exclusive with that of SRF owing to their overlapping base contacts. The cooperative promoter binding by SRF, however, can effectively displace prebound F-ACT1. In addition, an intact F-ACT1 binding site acts as a negative promoter element by restricting developmentally timed expression in myoblasts. F-ACT1 may therefore act as a repressor of skeletal alpha-actin gene transcription. This interplay between F-ACT1 and SRF may constitute a developmental as well as a physiologically regulated mechanism which modulates sarcomeric actin gene expression.

Activation of skeletal alpha-actin gene transcription: the cooperative formation of serum response factor-binding complexes over positive cis-acting promoter serum response elements displaces a negative-acting nuclear factor enriched in replicating myoblasts and nonmyogenic cells

Three upstream CBAR cis-acting promoter elements, containing the inner core CC(A/T)6GG of the serum response element (SRE), are required for myogenic cell type-restricted expression of the avian skeletal alpha-actin gene (K.L. Chow and R.J. Schwartz, Mol. Cell. Biol. 10:528-538, 1990). These actin SRE elements display differential binding properties with two distinct nuclear proteins, serum response factor (SRF) and another factor described here as F-ACT1. SRF is able to bind to all actin SREs with various affinities. This multisite interaction is marked by cooperative binding events in that the two high-affinity proximal and distal SREs facilitate the weak central-site interaction with SRF, leading to the formation of a higher-order SRF-promoter complex. Functional analyses reveal that undisrupted multiple SRF-DNA interactions are absolutely essential for promoter activity in myogenic cells. F-ACT1, present at higher levels in nonmyogenic cells and replicating myoblasts than in myotubes, binds solely to the proximal SRE, and its binding is mutually exclusive with that of SRF owing to their overlapping base contacts. The cooperative promoter binding by SRF, however, can effectively displace prebound F-ACT1. In addition, an intact F-ACT1 binding site acts as a negative promoter element by restricting developmentally timed expression in myoblasts. F-ACT1 may therefore act as a repressor of skeletal alpha-actin gene transcription. This interplay between F-ACT1 and SRF may constitute a developmental as well as a physiologically regulated mechanism which modulates sarcomeric actin gene expression.

Heterodimers of myogenic helix-loop-helix regulatory factors and E12 bind a complex element governing myogenic induction of the avian cardiac alpha-actin promoter

Recent studies have shown that two genes regulating myogenesis (MyoD and myogenin) are coexpressed with cardiac alpha-actin during early stages of skeletal muscle development. Myogenin and MyoD are members of a family of regulatory proteins which share a helix-loop-helix (HLH) motif required for dimerization and DNA binding. Myogenin and MyoD form heterodimers with the ubiquitous HLH protein E12 which bind cis-acting DNA elements that have an E box (CANNTG) at their core. E boxes are present in the control regions of numerous muscle-specific genes, although their functional importance in regulating many of these genes has not yet been evaluated. In this report we examine the possibility that myogenin (or MyoD) directly transactivates the cardiac alpha-actin promoter. Heterodimers of myogenin and E12 (or MyoD and E12) specifically bound a restriction fragment extending from -200 to -103 relative to the start of cardiac alpha-actin transcription. Methylation interference footprints pinpointed the site of interaction to an E box immediately adjacent to a previously identified CArG box (CArG3). Site-directed mutations to the DNA-binding site revealed that either an intact E box or an intact CArG3 is required for induction of the cardiac alpha-actin promoter in myoblasts and for transactivation by myogenin in cotransfected fibroblasts. However, deletion and substitution experiments indicate that the complex E box/CArG3 element alone does not confer muscle-specific expression to a minimal promoter. These results suggest that direct and indirect pathways involving multiple cis-acting elements mediate the induction of the cardiac alpha-actin promoter by myogenin and MyoD.

Heterodimers of myogenic helix-loop-helix regulatory factors and E12 bind a complex element governing myogenic induction of the avian cardiac alpha-actin promoter

Recent studies have shown that two genes regulating myogenesis (MyoD and myogenin) are coexpressed with cardiac alpha-actin during early stages of skeletal muscle development. Myogenin and MyoD are members of a family of regulatory proteins which share a helix-loop-helix (HLH) motif required for dimerization and DNA binding. Myogenin and MyoD form heterodimers with the ubiquitous HLH protein E12 which bind cis-acting DNA elements that have an E box (CANNTG) at their core. E boxes are present in the control regions of numerous muscle-specific genes, although their functional importance in regulating many of these genes has not yet been evaluated. In this report we examine the possibility that myogenin (or MyoD) directly transactivates the cardiac alpha-actin promoter. Heterodimers of myogenin and E12 (or MyoD and E12) specifically bound a restriction fragment extending from -200 to -103 relative to the start of cardiac alpha-actin transcription. Methylation interference footprints pinpointed the site of interaction to an E box immediately adjacent to a previously identified CArG box (CArG3). Site-directed mutations to the DNA-binding site revealed that either an intact E box or an intact CArG3 is required for induction of the cardiac alpha-actin promoter in myoblasts and for transactivation by myogenin in cotransfected fibroblasts. However, deletion and substitution experiments indicate that the complex E box/CArG3 element alone does not confer muscle-specific expression to a minimal promoter. These results suggest that direct and indirect pathways involving multiple cis-acting elements mediate the induction of the cardiac alpha-actin promoter by myogenin and MyoD.

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.

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.

Differential control of tropomyosin mRNA levels during myogenesis suggests the existence of an isoform competition-autoregulatory compensation control mechanism

We have isolated tropomyosin cDNAs from human skeletal muscle and nonmuscle cDNA libraries and constructed gene-specific DNA probes for each of the four functional tropomyosin genes. These DNA probes were used to define the regulation of the corresponding mRNAs during the process of myogenesis. Tropomyosin regulation was compared with that of beta- and gamma-actin. No two striated muscle-specific tropomyosin mRNAs are coordinately accumulated during myogenesis nor in adult striated muscles. Similarly, no two nonmuscle tropomyosins are coordinately repressed during myogenesis. However, mRNAs encoding the 248 amino acid nonmuscle tropomyosins and beta- and gamma-actin are more persistent in adult skeletal muscle than those encoding the 284 amino acid nonmuscle tropomyosins. In particular, the nonmuscle tropomyosin Tm4 is expressed at similar levels in adult rat nonmuscle and striated muscle tissues. We conclude that each tropomyosin mRNA has its own unique determinants of accumulation and that the 248 amino acid nonmuscle tropomyosins may have a role in the architecture of the adult myofiber. The variable regulation of nonmuscle isoforms during myogenesis suggests that the different isoforms compete for inclusion into cellular structures and that compensating autoregulation of mRNA levels bring gene expression into alignment with the competitiveness of each individual gene product. Such an isoform competition-autoregulatory compensation mechanism would readily explain the unique regulation of each gene.

A combination of closely associated positive and negative cis-acting promoter elements regulates transcription of the skeletal alpha-actin gene

The chicken skeletal alpha-actin gene promoter region provides at least a 75-fold-greater transcriptional activity in muscle cells than in fibroblasts. The cis-acting sequences required for cell type-restricted expression within this 200-base-pair (bp) region were elucidated by chloramphenicol acetyltransferase assays of site-directed Bg/II linker-scanning mutations transiently transfected into primary cultures. Four positive cis-acting elements were identified and are required for efficient transcriptional activity in myogenic cells. These elements, conserved across vertebrate evolution, include the ATAAAA box (-24 bp), paired CCAAT-box-associated repeats (CBARs; at -83 bp and -127 bp), and the upstream T+A-rich regulatory sequence (at -176 bp). Basal transcriptional activity in fibroblasts was not as dependent on the upstream CBAR or regions of the upstream T+A-rich regulatory sequence. Transfection experiments provided evidence that positive regulatory factors required for alpha-actin expression in fibroblasts are limiting. In addition, negative cis-acting elements were detected and found closely associated with the G+C-rich sequences that surround the paired CBARs. Negative elements may have a role in restricting developmentally timed expression in myoblasts and appear to inhibit promoter activity in nonmyogenic cells. Cell type-specific expression of the skeletal alpha-actin gene promoter is regulated by combinatorial and possibly competitive interactions between multiple positive and negative cis-acting elements.

A combination of closely associated positive and negative cis-acting promoter elements regulates transcription of the skeletal alpha-actin gene

The chicken skeletal alpha-actin gene promoter region provides at least a 75-fold-greater transcriptional activity in muscle cells than in fibroblasts. The cis-acting sequences required for cell type-restricted expression within this 200-base-pair (bp) region were elucidated by chloramphenicol acetyltransferase assays of site-directed Bg/II linker-scanning mutations transiently transfected into primary cultures. Four positive cis-acting elements were identified and are required for efficient transcriptional activity in myogenic cells. These elements, conserved across vertebrate evolution, include the ATAAAA box (-24 bp), paired CCAAT-box-associated repeats (CBARs; at -83 bp and -127 bp), and the upstream T+A-rich regulatory sequence (at -176 bp). Basal transcriptional activity in fibroblasts was not as dependent on the upstream CBAR or regions of the upstream T+A-rich regulatory sequence. Transfection experiments provided evidence that positive regulatory factors required for alpha-actin expression in fibroblasts are limiting. In addition, negative cis-acting elements were detected and found closely associated with the G+C-rich sequences that surround the paired CBARs. Negative elements may have a role in restricting developmentally timed expression in myoblasts and appear to inhibit promoter activity in nonmyogenic cells. Cell type-specific expression of the skeletal alpha-actin gene promoter is regulated by combinatorial and possibly competitive interactions between multiple positive and negative cis-acting elements.

Cellular distribution of smooth muscle actins during mammalian embryogenesis: expression of the alpha-vascular but not the gamma-enteric isoform in differentiating striated myocytes

The cellular distribution of the alpha-vascular and gamma-enteric smooth muscle actin isoforms was analyzed in rat embryos from gestational day (gd) 8 through the first neonatal week by in situ antigen localization using isoactin specific monoclonal antibodies. The alpha-vascular actin isoform was first detected on gd 10 in discrete cells lining the embryonic vasculature. By gd 14, this isoform was also present in the inner layers of mesenchymal cells condensing around the developing airways and gut. The gamma-enteric actin, however, was not detected until gd 15 when cells surrounding the developing aorta, airways, and gut labeled with the gamma-enteric-specific probe. There was continued expression of these two actin isoforms in regions of developing smooth muscle through the remainder of gestation and first neonatal week at which time their distribution coincided with that found in the adult. In addition to developing smooth muscle, the alpha-vascular actin isoform was expressed in differentiating striated muscle cells. On gd 10, there was intense labeling with the alpha-vascular specific probe in developing myocardiocytes and, within 24 h, in somitic myotomal cells. Although significant levels of this smooth muscle actin were present in striated myocytes through gd 17, by the end of the first postnatal week, alpha-vascular actin was no longer detectable in either cardiac or skeletal muscle. Thus, the normal developmental sequence of striated muscle cells includes the transient expression of the alpha-vascular smooth muscle actin isoform. In contrast, the gamma-enteric smooth muscle actin was not detected at any time in embryonic striated muscle. The differential timing of appearance and distribution of these two smooth muscle isoforms indicates that their expression is independently regulated during development.

Temporal resolution and sequential expression of muscle-specific genes revealed by in situ hybridization

The expression of muscle-specific mRNAs was analyzed directly within individual cells by in situ hybridization to chicken skeletal myoblasts undergoing differentiation in vitro. The probes detected mRNAs for sarcomeric myosin heavy chain (MHC) or the skeletal, cardiac, and beta isoforms of actin. Precise information as to the expression of these genes in individual cells was obtained and correlated directly with analyses of cell morphology and interactions, cell cycle stage, and immunofluorescence detection of the corresponding proteins. Results demonstrate that mRNAs for the two major muscle-specific proteins, myosin and actin, are not synchronously activated at the time of cell fusion. The mRNA for alpha-cardiac actin (CAct), known to be the predominant embryonic actin isoform in muscle, is expressed prior to cell fusion and prior to the expression of any isoform of muscle MHC mRNA. MHC mRNA accumulates rapidly immediately after fusion, whereas skeletal actin mRNA is expressed only in larger myofibers. Single cells expressing CAct mRNA have a characteristic short bipolar morphology, are in terminal G1, and do not contain detectable levels of the corresponding protein. In a pattern of expression reciprocal to that of CAct mRNA, beta-actin mRNA diminishes to low or undetectable levels in myofibers and in cells of the morphotype which expresses CAct mRNA. Finally, the intracellular distribution of mRNAs for different actin isoforms was compared using nonisotopic detection of isoform-specific oligonucleotide probes. This work illustrates a generally valuable approach to the analysis of cell differentiation and gene expression which directly integrates molecular, morphological, biochemical, and cell cycle information on individual cells.

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.

Differentiation of cardiac myocytes after mitogen withdrawal exhibits three sequential states of the ventricular growth response

During cardiac myogenesis, ventricular muscle cells lose the capacity to proliferate soon after birth. It is unknown whether this developmental block to mitotic division and DNA replication might involve irreversible repression of the cellular oncogene c-myc. Ventricular myocytes from 2 d-old rats continued to differentiate in vitro during 15 d of mitogen withdrawal, as shown by the formation of cross-striations, increased proportion of the muscle isoenzyme of creatine kinase, stable expression of alpha-cardiac actin and myosin heavy chain mRNAs, and appropriate down-regulation of alpha-skeletal actin mRNA. After mitogen withdrawal for 2 d, serum evoked both DNA synthesis and mitotic division; after 7 d, DNA replication was uncoupled from cell division; after 15 d, DNA synthesis itself was markedly attentuated. These three distinct phenotypic states resemble the sequential properties of growth found in the neonatal rat heart in vivo. Despite failure to induce DNA replication or division after 15 d of mitogen withdrawal, serum elicited both c-myc and alpha-skeletal actin as found during hypertrophy of the intact heart. The results agree with previous evidence that one or more functional pathways that transduce the effects of serum factors may persist in older cardiac muscle cells, and indicate that irreversible down-regulation of c-myc cannot be the basis for the loss of growth responses.