Tissue-specific transcriptional control of alpha- and beta-tropomyosins in chicken muscle development

Our journey with François Gros

Gillian Butler-Browne began working on muscle at the Institut Pasteur in the laboratory of François Gros in 1978. She characterized the expression profile of different myosin isoforms during both human and rodent development. Vincent Mouly joined this laboratory for his PhD in 1982, and defined the different populations of myoblasts appearing during development in birds and then in humans. Together, they demonstrated the impact of the limit in proliferation of the precursor cells on the regenerative capacity of human skeletal muscle, and their group developed models to evaluate the regenerative potential of skeletal muscle in vitro, measuring the telomeric erosion, and identified the involvement of a stress pathway in the proliferative arrest of muscle progenitors. A platform to produce human immortalized muscle cell lines was the successful result of this research, initiated with François Gros and W. E. Wright. The in vivo regenerative potential of human muscle cells was evaluated by injection into muscles of immunodeficient mice. Their group in collaboration with the clinical team of Professor Jean Lacau St-Guily and Professor Sophie Perié completed a successful autologous myoblast transplantation clinical trial for Oculo-pharyngeal muscular dystrophy. This common scientific career was made possible thanks to the precious and always benevolent support of François Gros.

Sarcomeric TPM3α in developing chicken

Cloning and sequencing of various tropomyosin isoforms expressed in chickens have been described since the early 1980s. However, to the best of our knowledge, this is the first report on the molecular characterization and the expression of the sarcomeric isoform of the TPM3 gene in cardiac and skeletal muscles from developing as well as adult chickens. Expression of TPM3α was performed by conventional RT-PCR as well as qRT-PCR using relative expression (by ΔCT as well as ΔΔCT methods) and by determining absolute copy number. The results employing all these methods show that the expression level of TPM3α is maximum in embryonic (10-day/15-day old) skeletal muscle and can barely be detected in both cardiac and skeletal muscles from the adult chicken. Our various RT-PCR analyses suggest that the expression of high molecular weight TPM3 isoforms are regulated at the transcription level from the proximal promoter at the 5'-end of the TPM3 gene.

Ex ovo electroporation for gene transfer into older chicken embryos

In ovo electroporation is an excellent method to ectopically induce or inhibit gene expression in chicken embryos and to study the in vivo function of genes during embryonic development. However, the application of electroporation in ovo to date is limited to an early stage of incubation (< stage 20). In older embryos (> stage 22), the vitelline and allantoic vessels have developed extensively and the in ovo manipulation of the embryo becomes exceedingly difficult. Therefore, in this study, we validate an ex ovo electroporation system, by which the time for performing electroporation can be extended up to at least day 7 of incubation. The application of this method will help to study gene function and regulation at later stages of development in the living chicken embryo.

Identification, characterization, and expression of a novel alpha-tropomyosin isoform in cardiac tissues in developing chicken

Tropomyosins are present in various muscle (skeletal, cardiac, and smooth) and non-muscle cells with different isoforms characteristic of specific cell types. We describe here a novel smooth/striated chimeric isoform that was expressed in developing chick heart in addition to the classically described TM-4 type. This novel alpha-Tm tropomyosin isoform, designated as alpha-Tm-2, contains exon 2a (in place of exon 2b). The known striated muscle isoform (alpha-Tm-1) was also expressed in embryonic hearts along with the striated muscle isoform of TM-4. In adult heart, TM-4 was expressed, however, expression of both alpha-Tm-1 and alpha-Tm-2 isoforms was drastically reduced or downregulated. Interestingly, we were unable to detect the expression of alpha-Tm-2 in embryonic and adult skeletal muscle, however, the alpha-Tm-1 isoform is expressed in embryonic and adult skeletal muscle. Examination of other possible isoforms of the alpha-TM gene, i.e., alpha-smooth muscle tropomyosin (alpha-Sm), alpha-Fibroblast-1 (alpha-F1), and alpha-Fibroblast-2 (alpha-F2) revealed expression in embryonic hearts and a significant reduction of each of these isoforms in adult heart. In order to elucidate the role of the newly discovered tropomyosin isoform in chicken, we ectopically expressed the GFP fusion protein of alpha-Tm-1 and alpha-Tm-2 separately into cardiomyocytes isolated from neonatal rats. Each isoform was incorporated into organized myofibrils. Our results suggest that the alpha-TM gene may undergo both positive and negative transcriptional control in chicken hearts during development.

Protein binding by the 3' untranslated region of alpha-striated tropomyosin

Tropomyosin is a component protein of the thin filament system in striated muscle, regulating the interaction between actin and myosin. The 3' untranslated region of the alpha-striated tropomyosin gene (TM UTR) induces muscle differentiation when expressed in primary fibroblasts, but the mechanism has not been defined. We hypothesize that fibroblasts utilize resident proteins to effect this response, perhaps by TM UTR binding to protein(s). In order to facilitate identification of protein(s) involved in mediating this differentiation response, we investigated the potential for this sequence to bind to cellular protein utilizing electrophoretic mobility gel shifting analysis (EMSA) with and without UV cross-linking. Under very specific conditions (including pH, KCl, and Mg concentration and extent of phosphorylation of protein), the TM UTR is able to bind protein in cells that differentiate upon TM UTR expression. Protein binding is significantly more extensive in cytoplasmic than nuclear protein preparations. Secondary structure of the RNA probe facilitates protein binding. The molecular masses of bound proteins are approximately 42 and 115 kDa under basal conditions. EMSA analysis of extract from cultured skeletal muscle confirms that protein binding by the TM UTR occurs in this cell type, and is more extensive in less differentiated cells. The demonstration of highly regulated protein binding by the TM UTR raises the possibility that this sequence may cause differentiation by binding to endogenous proteins, and further that this sequence may play a role in normal differentiation. Identification of proteins bound by the TM UTR will be necessary to completely define the mechanism by which it causes differentiation.

Identification of a novel tropomodulin isoform, skeletal tropomodulin, that caps actin filament pointed ends in fast skeletal muscle

Tropomodulin (E-Tmod) is an actin filament pointed end capping protein that maintains the length of the sarcomeric actin filaments in striated muscle. Here, we describe the identification and characterization of a novel tropomodulin isoform, skeletal tropomodulin (Sk-Tmod) from chickens. Sk-Tmod is 62% identical in amino acid sequence to the previously described chicken E-Tmod and is the product of a different gene. Sk-Tmod isoform sequences are highly conserved across vertebrates and constitute an independent group in the tropomodulin family. In vitro, chicken Sk-Tmod caps actin and tropomyosin-actin filament pointed ends to the same extent as does chicken E-Tmod. However, E- and Sk-Tmods differ in their tissue distribution; Sk-Tmod predominates in fast skeletal muscle fibers, lens, and erythrocytes, while E-Tmod is found in heart and slow skeletal muscle fibers. Additionally, their expression is developmentally regulated during chicken breast muscle differentiation with Sk-Tmod replacing E-Tmod after hatching. Finally, in skeletal muscle fibers that coexpress both Sk- and E-Tmod, they are recruited to different actin filament-containing cytoskeletal structures within the cell: myofibrils and costameres, respectively. All together, these observations support the hypothesis that vertebrates have acquired different tropomodulin isoforms that play distinct roles in vivo.

Molecular and physiological effects of overexpressing striated muscle beta-tropomyosin in the adult murine heart

Tropomyosins comprise a family of actin-binding proteins that are central to the control of calcium-regulated striated muscle contraction. To understand the functional role of tropomyosin isoform differences in cardiac muscle, we generated transgenic mice that overexpress striated muscle-specific beta-tropomyosin in the adult heart. Nine transgenic lines show a 150-fold increase in beta-tropomyosin mRNA expression in the heart, along with a 34-fold increase in the associated protein. This increase in beta-tropomyosin message and protein causes a concomitant decrease in the level of alpha-tropomyosin transcripts and their associated protein. There is a preferential formation of the alpha beta-heterodimer in the transgenic mouse myofibrils, and there are no detectable alterations in the expression of other contractile protein genes, including the endogenous beta-tropomyosin isoform. When expression from the beta-tropomyosin transgene is terminated, alpha-tropomyosin expression returns to normal levels. No structural changes were observed in these transgenic hearts nor in the associated sarcomeres. Interestingly, physiological analyses of these hearts using a work-performing model reveal a significant effect on diastolic function. As such, this study demonstrates that a coordinate regulatory mechanism exists between alpha- and beta-tropomyosin gene expression in the murine heart, which results in a functional correlation between alpha- and beta-tropomyosin isoform content and cardiac performance.

Promoter elements and transcriptional control of the chicken tropomyosin gene [corrected]

The chicken beta tropomyosin (beta TM) gene has two alternative transcription start sites (sk and nmCAP sites) which are used in muscle or non muscle tissues respectively. In order to understand the mechanisms involved in the tissue-specific and developmentally-regulated expression of the beta TM gene, we have analyzed the 5' regions associated with each CAP site. Truncated regions 5' to the nmCAP site were inserted upstream to the bacterial chloramphenicol acetyltransferase (CAT) reporter gene and these constructs were transfected into avian myogenic and non myogenic cells. The maximum transcription is driven by the CAT construct (-168/ + 216 nt) in all cell types. Previous deletion analysis of the region 5' to the beta TMskCAP site has indicated that 805 nt confer myotube-specific transcription. In this work, we characterized an enhancer element (-201/-68 nt) which contains an E box (-177), a variant CArG box (-104) and a stretch of 7Cs (-147). Mutation of any of these motifs results in a decrease of the myotube-specific transcriptional activity. Electrophoretic mobility shift assays indicate that these cis-acting sequences specifically bind nuclear proteins. This enhancer functions in an orientation-dependent manner.

Differential regulation of skeletal muscle myosin-II and brush border myosin-I enzymology and mechanochemistry by bacterially produced tropomyosin isoforms

In this report, we have compared the physical properties and actin-binding characteristics of several bacterially produced nonmuscle and striated muscle tropomyosins, and we have examined the effects of these isoforms on the interactions of actin with two structurally distinct classes of myosin: striated muscle myosin-II and brush border (BB) myosin-I. All of the bacterially produced nonmuscle tropomyosins bind to F-actin with the expected stoichiometry and with affinities comparable to that of a tissue produced alpha-tropomyosin, although the striated muscle tropomyosin CTm7 has a lower affinity for F-actin than a tissue-purified striated muscle alpha tropomyosin. The bacterially produced isoforms also protect F-actin from severing by villin as effectively as tissue-purified striated muscle alpha-tropomyosin. The bacterially produced 284 amino acid striated muscle tropomyosin isoform CTm7, the 284 amino acid nonmuscle tropomyosin isoform CTm4, and two chimeric tropomyosins (CTm47 and CTm74) all inhibit the actin-activated MgATPase activity of muscle myosin S1 by approximately 70-85%, comparable to the inhibition seen with tissue-purified striated muscle alpha tropomyosin. The 248 amino acid tropomyosin XTm4 stimulated the actin-activated MgATPase activity of muscle myosin S1 approximately two- to threefold. The in vitro sliding of actin filaments translocated by muscle myosin-II (2.4 microns/sec at 19 degrees C, 5.0 microns/s at 24 degrees C) increased 25-65% in the presence of XTm4. Tropomyosins CTm4, CTm7, CTm47, and CTm74 had no detectable effect on myosin-II motility. The actin-activated MgATPase activity of BB myosin-I was inhibited 75-90% by all of the tropomyosin isoforms tested, including the 248 amino acid tropomyosin XTm4. BB myosin-I motility (50 nm/s) was completely inhibited by both the 248 and 284 amino acid tropomyosins. These results demonstrate that bacterially produced tropomyosins can differentially regulate myosin enzymology and mechanochemistry, and suggest a role for tropomyosin in the coordinated regulation of myosin isoforms in vivo.

Denervated chicken breast muscle displays discoordinate regulation and differential patterns of expression of alpha f and beta tropomyosin genes

The expression of the alpha fast (alpha f) and beta tropomyosin (TM) genes has been analysed with muscle-specific and common cDNA probes after unilateral nerve section of the pectoralis major muscle (PM) in 4-week-old chickens. The following were observed in denervated muscles. (1) The beta TM mRNA, which was repressed during development, reaccumulates in a biphasic curve with the increase in the beta TM protein lagging behind the changes in its mRNA. Accordingly, no beta TM is seen in products translated in vitro from total and polyA+ RNA obtained 1 week after denervation. No such translation block is seen with RNA obtained from control or muscles denervated for 6 weeks. (2) No changes in the alpha fTM mRNA and corresponding protein are observed. (3) RNA processing of the two genes is not changed. (4) In the contralateral muscles, transitory increases in alpha f and beta TM mRNAs are observed while the corresponding proteins remain unchanged. Our data suggest that muscle fibres display early and long-term responses to the loss of neural input which might result from a combination of changes produced by regenerative processes and reprogramming of existing fibres. Moreover, in contrast to normal development, no reciprocal changes of alpha f and beta TM expression are seen in denervated muscles.

Identification of a program of contractile protein gene expression initiated upon skeletal muscle differentiation

The functional diversity of skeletal muscle is largely determined by the combinations of contractile protein isoforms that are expressed in different fibers. Just how the developmental expression of this large array of genes is regulated to give functional phenotypes is thus of great interest. In the present study, we performed a comprehensive analysis of contractile protein isoform mRNA profiles in skeletal muscle systems representing each generation of fiber formed: primary, secondary, and regenerating fibers. We find that in each system examined there is a common pattern of isoform gene expression during early differentiation for 5 of the 6 gene families we have investigated: myosin light chain (MLC)1, MLC2, tropomyosin, troponin (Tn)C, and TnI. We suggest that the common isoform patterns observed together represent a genetic program of skeletal muscle differentiation that is independent of the mature fiber phenotype and is found in all newly formed myotubes. Within each of these contractile protein gene families the program is independent of the isoforms of myosin heavy chain (MHC) expressed. The maintenance of such a program may reflect a specific requirement of the initial differentiation process.

Contractile protein isoforms in muscle development

The contractile proteins of skeletal muscle are often represented by families of very similar isoforms. Protein isoforms can result from the differential expression of multigene families or from multiple transcripts from a single gene via alternative splicing. In many cases the regulatory mechanisms that determine the accumulation of specific isoforms via alternative splicing or differential gene expression are being unraveled. However, the functional significance of expressing different proteins during muscle development remains a key issue that has not been resolved. It is widely believed that distinct isoforms within a family are uniquely adapted to muscles with different physiological properties, since separate isoform families are often coordinately regulated within functionally distinct muscle fiber types. It is also possible that different isoforms are functionally indistinguishable and represent an inherent genetic redundancy among critically important muscle proteins. The goal of this review is to assess the evidence that muscle proteins which exist as different isoforms in developing and mature skeletal and cardiac muscles are functionally unique. Since regulation of both transcription and alternative splicing within multigene families may also be an important factor determining the accumulation of specific protein isoforms, evidence that genetic regulation rather than protein coding information provides the functional basis of isoform diversity is also examined.

The proximal promoter of the aldolase A gene remains active during myogenesis in vitro and muscle development in vivo

The gene for aldolase A in mouse has been shown to be regulated by alternative promoters with attendant alternative first exons. The distal promoter/exon M functions only in muscle while the proximal promoter/exon H is active in early muscle development and in most other tissues. We have analyzed the developmental expression of M and H promoters in mouse throughout myogenesis both in vitro and in vivo. In C2C12 cells RNase protection assays revealed the M promoter is induced within 24 hr of the onset of myogenic differentiation, and both M- and H-specific mRNAs accumulate over 5 days in culture. Nuclear run-on transcription and in situ hybridization with an exon-specific probe demonstrate that the H promoter remains transcriptionally active even in differentiated myotubes. The in vitro results were then compared to similar RNase protection studies of M and H expression during muscle development in vivo. These data show a marked similarity between promoter activation and steady-state transcript accumulation in vivo and in vitro, but within a limited developmental time frame (E15 to 1 week postnatal). In situ hybridizations suggest that simultaneous transcription from both promoters may also occur early in muscle development. Furthermore, the M promoter shows no fiber-type restriction until 1 to 3 weeks postnatally, coincident with muscle maturation, while the H promoter remains transcriptionally active at all stages of development and in all fiber types.

Actin and myosin genes are transcriptionally regulated during mouse skeletal muscle development

During primary and secondary myotube formation in utero and subsequent maturation of muscle fibers after birth there are complex changes in the pattern of contractile protein gene expression at the RNA and protein levels. In order to determine the degree of transcriptional regulation of actin and myosin genes we have carried out "nuclear run-on" experiments using nuclei prepared from the limb muscle of mice at 14.5, 15.5, 17.5, and 18.5 days in utero and at 10-12 and 12.5 days after birth. We show that transitions in the expression of these genes in vivo are regulated transcriptionally. Transcription of the sarcomeric alpha-actins changes from cardiac to predominantly skeletal actin over this time period; transcription of the beta-actin gene is repressed. The myosin heavy chain and myosin light chain genes also undergo transcriptional transitions during muscle development. Notably, transcription from the MLC3F promoter is activated after that of the MLC1F promoter, which is part of the same gene. These results are discussed in the context of published RNA data.

Biphasic expression of slow myosin light chains and slow tropomyosin isoforms during the development of the human quadriceps muscle

Using a two-dimensional electrophoresis technique coupled with sensitive silver staining, we have investigated the chronology of appearance of the myosin light chain and tropomyosin isoforms during early stages of human quadriceps development. Our results show that slow myosin light chains and the slow tropomyosin isoform are not detected at 6 weeks of gestation. These isoforms transiently appear between 12.5 weeks and 15 weeks of gestation and then disappear. The slow myosin light chains are re-expressed at 31 weeks of gestation and the slow tropomyosin isoform later at 36 weeks of gestation, and normally remained expressed into the adulthood. Our study thus reveals a biphasic expression of the slow myosin light chains and the slow tropomyosin isoform in developing human quadriceps muscle.

Isolation of transcriptionally active nuclei from striated muscle using Percoll density gradients

A procedure is described utilizing Percoll density media for the separation of nuclei and myofibrils in homogenates of adult skeletal muscle. Using this method, transcriptionally active nuclei can be readily obtained in relatively high yield (approximately 30%). In vitro RNA polymerase run-on-labeled RNAs isolated from these nuclei can be used in hybridization assays to study the transcriptional activities of specific genes. Percoll density gradient centrifugation should be useful for the isolation of nuclei from a variety of other tissues in which, like skeletal muscle, subcellular or tissue components cosediment with nuclei in conventional sucrose density centrifugation.

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.