Delineation of a slow-twitch-myofiber-specific transcriptional element by using in vivo somatic gene transfer

DNA methylation assessment from human slow- and fast-twitch skeletal muscle fibers

A new application of the reduced representation bisulfite sequencing method was developed using low-DNA input to investigate the epigenetic profile of human slow- and fast-twitch skeletal muscle fibers. Successful library construction was completed with as little as 15 ng of DNA, and high-quality sequencing data were obtained with 32 ng of DNA. Analysis identified 143,160 differentially methylated CpG sites across 14,046 genes. In both fiber types, selected genes predominantly expressed in slow or fast fibers were hypomethylated, which was supported by the RNA-sequencing analysis. These are the first fiber type-specific methylation data from human skeletal muscle and provide a unique platform for future research.NEW & NOTEWORTHY This study validates a low-DNA input reduced representation bisulfite sequencing method for human muscle biopsy samples to investigate the methylation patterns at a fiber type-specific level. These are the first fiber type-specific methylation data reported from human skeletal muscle and thus provide initial insight into basal state differences in myosin heavy chain I and IIa muscle fibers among young, healthy men.

Molecular cloning and comparative characterization of the porcine troponin I family

Troponin I (TnI) is a family of three muscle-specific myofibrillar proteins involved in calcium-sensitive regulation of contraction in cardiac and skeletal muscle. In this study, the full-length cDNA and genomic sequence of three genes of porcine TnI family were cloned and sequenced. The full-length cDNA of TNNI1, TNNI2, and TNNI3 genes were 989 bp, 734 bp, and 831 bp in length, which contained an open reading frame of 564, 549, and 636 nucleotides, respectively. Three Troponin I shared 54.4 approximately 58.3% similarity with each other in their predicted amino acid sequences. The TNNI1, TNNI2, and TNNI3 displayed the same genomic structure as other vertebrates and spanned over 9785 bp, 2373 bp, and 3648 bp genomic regions, respectively. The regulatory elements in the proximal promoter of TNNI2 and TNNI3 were conserved among human, mouse, and pig, but regulatory element differences existed in the TNNI1 promoter among them. Expression profiling showed that TnI genes were widely expressed in the tissues studied, with the highest expression level of TNNI1 and TNNI2 in skeletal muscle, and TNNI3 in cardiac muscle.

Direct visualisation and kinetic analysis of normal and nemaline myopathy actin polymerisation using total internal reflection microscopy

Actin filaments were formed by elongation of pre-formed nuclei (short crosslinked actin-HMM complexes) that were attached to a microscope cover glass. By using TIRF illumination we could see actin filaments at high contrast despite the presence of 150 nM TRITC-phalloidin in the solution. Actin filaments showed rapid bending and translational movements due to Brownian motion but the presence of the methylcellulose polymer network constrained lateral movement away from the surface. Both the length and the number of filaments increased with time. Some filaments did not change length at all and some filaments joined up end-to-end (annealing). We did not see any decrease in filament length or filament breakage. For quantitative analysis of polymerisation time course we measured the contour length of all the filaments in a frame at a series of time points and also tracked the length of individual filaments over time. Elongation rate was the same measured by both methods (0.23 microm/min at 0.1 microM actin) and was up to 10 times faster than previously published measurements. The annealed filament population reached 30% of the total after 40 min. Polymerisation rate increased linearly with actin concentration. K(on) was 2.07 microm min(-1) microM(-1) (equivalent to 34.5 monomers s(-1) microM(-1)) and critical concentration was less than 20 nM. This technique was used to study polymerisation of a mutant actin (D286G) from a transgenic mouse model. D286G actin elongated at a 40% lower rate than non-transgenic actin.

Functional characterization of mouse fetal TnI gene promoters in myocardial cells

Two major troponin I (TnI) genes, fetal TnI (ssTnI) and adult TnI (cTnI), are expressed in the mammalian heart under the control of a developmentally regulated program. In this study, the up-stream domain ( approximately 1,800 bp) of mouse fetal TnI gene has been cloned and characterized. There is a high homology of this region among mouse, rat and human. Analysis of the sequence revealed several putative regulatory domains and binding sites (Sp1 binding sites, GATA binding site, MyoD, CREB, MEF2, AP1, NFkappaB, etc). Transfection assays indicated that conserved GA-rich sequences, CREB and a CCAAT box within the first 300 bp upstream of the transcription start site were critical for the gene expression. Electrophoretic mobility shift assays (EMSAs) and chromatin immunoprecipitation (ChIP) assays revealed binding proteins to CREB site in nuclear extracts from myocardial cells. An inhibitory domain was revealed within the sequence between -1,700 to -1,780. Thyroid hormone (T(3)) caused a significant inhibitory effect on ssTnI expression in myocardial cells whereas this effect was not evident in CHO cells.

Glycogen synthase kinase 3 suppresses myogenic differentiation through negative regulation of NFATc3

Skeletal muscle atrophy is a prominent and disabling feature in many chronic diseases. Prevention or reversal of muscle atrophy by stimulation of skeletal muscle growth could be an important therapeutic strategy. Glycogen synthase kinase 3beta (GSK-3beta) has been implicated in the negative regulation of skeletal muscle growth. Since myogenic differentiation is an essential part of muscle growth, we investigated if inhibition of GSK-3beta is sufficient to stimulate myogenic differentiation and whether this depended on regulation of the transcription factor nuclear factor of activated T-cells (NFAT). In both myogenically converted mouse embryonic fibroblasts and C2C12 myoblasts, deficiency of GSK-3beta protein (activity) resulted in enhanced myotube formation and muscle-specific gene expression during differentiation, which was reversed by reintroduction of wild type but not kinase-inactive (K85R) GSK-3beta. In addition, GSK-3beta inhibition restored myogenic differentiation following calcineurin blockade, which suggested the involvement of NFAT. GSK-3beta-deficient mouse embryonic fibroblasts or myoblasts displayed enhanced nuclear translocation of NFATc3 and elevated NFAT-sensitive promoter transactivation, which was reduced by reintroducing wild type, but not K85R GSK-3beta. Overexpression of NFATc3 increased muscle gene promoter transactivation, which was abolished by co-expression of wild type GSK-3beta. Finally, stimulation of muscle gene expression observed following GSK-3beta inhibition was strongly attenuated in NFATc3-deficient myoblasts, indicating that this response requires NFATc3. Collectively, our data demonstrate negative regulation of myogenic differentiation by GSK-3beta through a transcriptional mechanism that depends on NFATc3. Inhibition of GSK-3beta may be a potential strategy in prevention or treatment of muscle atrophy.

IIx myosin heavy chain promoter regulation cannot be characterized in vivo by direct gene transfer

In skeletal muscle of the adult mammal IIx is a pivotal myosin heavy chain (MHC) isoform that can be either up- or downregulated depending on both the fiber type of the target muscle and the type of external stimulus imposed. Since little is known about promoter elements of the IIx MHC gene that are important for its transcriptional regulation in vivo,the main goal of this study was to characterize IIx MHC promoter activity and identify potential regulatory elements on the IIx MHC promoter. A direct gene transfer approach was used, and this approach involved transfection of promoter-reporter constructs into intact rat soleus and plantaris muscle under control and denervated conditions, as well as hindlimb suspension (i.e., models to upregulate IIx MHC transcription). Fast-twitch (plantaris) muscle fibers were confirmed to have significantly greater IIx MHC transcriptional products (pre-mRNA and mRNA) than slow-twitch (soleus) muscle fibers. However, promoter sequences corresponding to −2671 to +1720, −1000 to +392, and −605/+392 relative to the IIx MHC transcription start site, plus an additional construct ligated to a putative embryonic MHC enhancer, failed to produce a fiber type-specific response that is characteristic of the endogenous IIx MHC promoter. Furthermore, the activity of these promoter constructs did not demonstrate the expected response to denervation or hindlimb suspension (i.e., marked upregulation), despite normal uptake and activity of a coinjected α-actin reference promoter. On the basis of these findings with IIx MHC promoter-reporters we conclude that the loss of the native chromatin environment as well as other necessary cis elements may preclude use of the gene transfer approach, thereby suggesting that there are hidden layers of regulation for the IIx MHC gene.

MusTRD can regulate postnatal fiber-specific expression

Human MusTRD1alpha1 was isolated as a result of its ability to bind a critical element within the Troponin I slow upstream enhancer (TnIslow USE) and was predicted to be a regulator of slow fiber-specific genes. To test this hypothesis in vivo, we generated transgenic mice expressing hMusTRD1alpha1 in skeletal muscle. Adult transgenic mice show a complete loss of slow fibers and a concomitant replacement by fast IIA fibers, resulting in postural muscle weakness. However, developmental analysis demonstrates that transgene expression has no impact on embryonic patterning of slow fibers but causes a gradual postnatal slow to fast fiber conversion. This conversion was underpinned by a demonstrable repression of many slow fiber-specific genes, whereas fast fiber-specific gene expression was either unchanged or enhanced. These data are consistent with our initial predictions for hMusTRD1alpha1 and suggest that slow fiber genes contain a specific common regulatory element that can be targeted by MusTRD proteins.

Targeted inhibition of Ca2+/calmodulin signaling exacerbates the dystrophic phenotype in mdx mouse muscle

In this study, we crossbred mdx mice with transgenic mice expressing a small peptide inhibitor for calmodulin (CaM), known as the CaM-binding protein (CaMBP), driven by the slow fiber-specific troponin I slow promoter. This strategy allowed us to determine the impact of interfering with Ca(2+)/CaM-based signaling in dystrophin-deficient slow myofibers. Consistent with impairments in the Ca(2+)/CaM-regulated enzymes calcineurin and Ca(2+)/CaM-dependent kinase, the nuclear accumulation of nuclear factor of activated T-cell c1 and myocyte enhancer factor 2C was reduced in slow fibers from mdx/CaMBP mice. We also detected significant reductions in the levels of peroxisome proliferator gamma co-activator 1alpha and GA-binding protein alpha mRNAs in slow fiber-rich soleus muscles of mdx/CaMBP mice. In parallel, we observed significantly lower expression of myosin heavy chain I mRNA in mdx/CaMBP soleus muscles. This correlated with fiber-type shifts towards a faster phenotype. Examination of mdx/CaMBP slow muscle fibers revealed significant reductions in A-utrophin, a therapeutically relevant protein that can compensate for the lack of dystrophin in skeletal muscle. In accordance with lower levels of A-utrophin, we noted a clear exacerbation of the dystrophic phenotype in mdx/CaMBP slow fibers as exemplified by several pathological indices. These results firmly establish Ca(2+)/CaM-based signaling as key to regulating expression of A-utrophin in muscle. Furthermore, this study illustrates the therapeutic potential of using targets of Ca(2+)/CaM-based signaling as a strategy for treating Duchenne muscular dystrophy (DMD). Finally, our results further support the concept that strategies aimed at promoting the slow oxidative myofiber program in muscle may be effective in altering the relentless progression of DMD.

Control of slow myosin heavy chain 2 gene expression by glycogen synthase kinase activity in skeletal muscle fibers

Skeletal muscle fiber type and expression of slow muscle fiber type specific genes are regulated by fiber type specific cell signaling events initiated by innervation. In avian muscle fibers, expression of the slow myosin heavy chain 2 (MyHC2) gene defines fast versus slow muscle fiber types, and its expression is dependent on the transcription factor, nuclear factor of activated T cells (NFAT). Glycogen synthase kinase 3 (GSK3) phosphorylates NFAT and inhibits its transactivating potential. We report here that expression of the slow MyHC2 gene is dependent on GSK3 activity. Inhibition of GSK3 activity by SB216763 or LiCl induced expression of the slow MyHC2 gene in non-innervated medial adductor (MA) muscle fibers and in innervated fast pectoralis major (PM) muscle fibers. Innervation of MA and PM muscle fibers did not significantly alter GSK3 activity. However, inhibition of GSK3 activity increased NFAT-mediated transcriptional activity, required for full activation of the slow MyHC2 gene, and overexpression of GSK3 reduced NFAT-mediated transcription. Inhibition of GSK3 activity was sufficient to induce slow MyHC2 gene expression in non-innervated MA muscle fibers but not in non-innervated PM muscle fibers, suggesting that fiber type specific mechanisms differentially regulate slow MyHC2 gene expression in innervated muscle fibers.

Molecular analysis of fiber type-specific expression of murine myostatin promoter

Myostatin is a negative regulator of muscle growth, and absence of the functional myostatin protein leads to the heavy muscle phenotype in both mouse and cattle. Although the role of myostatin in controlling muscle mass is established, little is known of the mechanisms regulating the expression of the myostatin gene. In this study, we have characterized the murine myostatin promoter in vivo. Various constructs of the murine myostatin promoter were injected into the quadriceps muscle of mice, and the reporter luciferase activity was analyzed. The results indicate that of the seven E-boxes present in the 2.5-kb fragment of the murine myostatin promoter, the E5 E-box plays an important role in the regulation of promoter activity in vivo. Furthermore, the in vitro studies demonstrated that MyoD preferentially binds and upregulates the murine myostatin promoter activity. We also analyzed the activity of the bovine and murine promoters in murine skeletal muscle and showed that, despite displaying comparable levels of activity in murine myoblast cultures, bovine myostatin promoter activity is much weaker than murine myostatin promoter in mice. Finally, we demonstrate that in vivo, the 2.5-kb region of the murine myostatin promoter is sufficient to drive the activity of the reporter gene in a fiber type-specific manner.

Insulin-like growth factor-induced transcriptional activity of the skeletal alpha-actin gene is regulated by signaling mechanisms linked to voltage-gated calcium channels during myoblast differentiation

IGF-I activates signaling pathways that increase the expression of muscle-specific genes in differentiating myoblasts. Induction of skeletal alpha-actin expression occurs during differentiation through unknown mechanisms. The purpose of this investigation was to examine the mechanisms that IGF-I uses to induce skeletal alpha-actin gene expression in C2C12 myoblasts. IGF-I increased skeletal alpha-actin promoter activity by 107% compared with the control condition. Ni(+) [T-type voltage-gated Ca(2+) channel (VGCC) inhibitor] reduced basal-induced activation of the skeletal alpha-actin promoter by approximately 84%, and nifedipine (L-type VGCC inhibitor) inhibited IGF-I-induced activation of the skeletal alpha-actin promoter by 29-48%. IGF-I failed to increase skeletal alpha-actin promoter activity in differentiating dysgenic (lack functional L-type VGCC) myoblasts; 30 mm K(+) and 30 mm K(+)+IGF-I increased skeletal alpha-actin promoter activity by 162% and 76% compared with non-IGF-I or IGF-I-only conditions, respectively. IGF-I increased calcineurin activity, which was inhibited by cyclosporine A. Further, cyclosporine A inhibited K(+)+IGF-I-induced activation of the skeletal alpha-actin promoter. Constitutively active calcineurin increased skeletal alpha-actin promoter activity by 154% and rescued the nifedipine-induced inhibition of L-type VGCC but failed to rescue the Ni(+)-inhibition of T-type VGCC. IGF-I-induced nuclear factor of activated T-cells transcriptional activity was not inhibited by nifedipine or Ni(+). IGF-I failed to increase serum response factor transcriptional activity; however, serum response factor activity was reduced in the presence of Ni(+). These data suggest that IGF-I-induced activation of the skeletal alpha-actin promoter is regulated by the L-type VGCC and calcineurin but independent of nuclear factor of activated T-cell transcriptional activity as C2C12 myoblasts differentiate into myotubes.

Innervation-dependent and fiber type-specific transcriptional regulation of the slow myosin heavy chain 2 promoter in avian skeletal muscle fibers

Skeletal muscle fiber type is regulated, in part, by innervation leading to transcriptional regulation of fiber type-specific genes. Here, we report the initial characterization of the transcriptional regulation of the slow myosin heavy chain 2 (MyHC2) promoter in innervated and noninnervated slow medial adductor (MA) and fast pectoralis major (PM) muscle fibers in cell culture. The proximal 1358 bp of slow MyHC2 upstream DNA contains a functional E-box and binding sites for myocyte enhancer factor 2 (MEF2) and nuclear factor of activated T cells (NFAT). Mutagenesis studies indicated that both MEF2 and NFAT binding sites are required for innervation-induced slow MyHC2 promoter activity in MA muscle fibers. However, MEF2 transcription factor activity was unaffected by innervation and did not demonstrate fiber type-specific interactions with the slow MyHC2 MEF2 binding site. NFAT transcription factor activity did increase in innervated MA muscle fibers and not in PM muscle fibers, indicating innervation and muscle fiber type-specific regulation. However, transfection of constitutively active NFAT indicated that NFAT is insufficient to induce slow MyHC2 gene expression in either fast PM or slow MA muscle fibers without innervation. These results indicate the requirement for MEF2 and NFAT in innervation-induced slow MyHC2 gene expression and suggest that additional innervation-dependent and fiber type-specific control of slow MyHC2 gene expression resides in MA and PM muscle fibers, respectively.

Muscle electrotransfer as a tool for studying muscle fiber-specific and nerve-dependent activity of promoters

Muscle electrotransfer has recently become a promising tool for efficient delivery of plasmids and transgene expression in skeletal muscle. This technology has been mainly applied to use of muscle as a bioreactor for production of therapeutic proteins. However, it remains to be determined whether muscle electrotransfer may also be accurately used as an alternative tool to transgenesis for studying aspects of muscle-specific gene control that must be explored in fully mature muscle fibers in vivo, such as fiber specificity and nerve dependence. It was also not known to what extent the initial electrical stimulations alter muscle physiology and gene expression. Therefore, optimized conditions of skeletal muscle electroporation were first tested for their effects on muscles of transgenic mice harboring a pM310-CAT transgene in which the CAT reporter gene was under control of the fast IIB fiber-specific and nerve-dependent aldolase A pM promoter. Surprisingly, electrostimulation led to a drastic but transient shutdown of pM310-CAT transgene expression concomitant with very transient activation of MyoD and, mostly, with activation of myogenin, suggesting profound alterations in transcriptional status of the electroporated muscle. Return to a normal transcriptional state was observed 7-10 days after electroporation. Therefore, we investigated whether a reporter construct placed under control of pM could exhibit fiber-specific expression 10 days after electrotransfer in either fast tibialis anterior or slow soleus muscle. We show that not only fiber specificity, but also nerve dependence, of a pM-driven construct can be reproduced. However, after electrotransfer, pM displayed a less tight control than previously observed for the same promoter when integrated in a chromatin context.

hMusTRD1alpha1 represses MEF2 activation of the troponin I slow enhancer

The novel transcription factor hMusTRD1alpha1 (human muscle TFII-I repeat domain-containing protein 1alpha1; previously named MusTRD1; O'Mahoney, J. V., Guven, K. L., Lin, J., Joya, J. E., Robinson, C. S., Wade, R. P., and Hardeman, E. C. (1998) Mol. Cell. Biol. 18, 6641-6652) was identified in a yeast one-hybrid screen as a protein that binds within an upstream enhancer-containing region of the skeletal muscle-specific gene, TNNI1 (human troponin I slow; hTnIslow). It has been proposed that hMusTRD1alpha1 may play an important role in fiber-specific muscle gene expression by virtue of its ability to bind to an Inr-like element (nucleotides -977 to -960) within the hTnIslow upstream enhancer-containing region that is necessary for slow fiber-specific expression. In this study we demonstrate that both MEF2C, a known regulator of slow fiber-specific genes, and hMusTRD1alpha1 regulate hTnIslow through the Inr-like element. Co-transfection assays in C2C12 cells and Cos-7 cells demonstrate that hMusTRD1alpha1 represses hTnIslow transcription and prevents MEF2C-mediated activation of hTnIslow transcription. Gel shift analysis shows that hMusTRD1alpha1 can abrogate MEF2C binding to its cognate site in the hTnIslow enhancer. Glutathione S-transferase pull-down assays demonstrate that hMusTRD1alpha1 can interact with both MEF2C and the nuclear receptor co-repressor. The data support the role of hMusTRD1alpha1 as a repressor of slow fiber-specific transcription through mechanisms involving direct interactions with MEF2C and the nuclear receptor co-repressor.

TnIfast IRE enhancer: multistep developmental regulation during skeletal muscle fiber type differentiation

To identify developmental steps leading to adult skeletal muscle fiber-type-specific gene expression, we carried out transgenic mouse studies of the IRE enhancer of the quail TnIfast gene. Histochemical analysis of IRE/herpesvirus tk promoter/beta-galactosidase reporter transgene expression in adult muscle directly demonstrated IRE-driven fast vs. slow fiber-type specificity, and IIB>IIX>IIA differential expression among the fast fiber types: patterns similar to those of native-promoter TnIfast constructs. These tissue- and cell-type specificities are autonomous to the IRE and do not depend on interactions with a muscle gene promoter. Developmental studies showed that the adult pattern of IRE-driven transgene expression emerges in three steps: (1) activation during the formation of primary embryonic (presumptive slow) muscle fibers; (2) activation, to markedly higher levels, during formation of secondary (presumptive fast) fibers, and (3) differential augmentation of expression during early postnatal maturation of the IIB, IIX, IIA fast fiber types. These results provide insight into the roles of gene activation and gene repression mechanisms in fiber-type specificity and can account for apparently disparate results obtained in previous studies of TnI isoform expression in development. Each of the three IRE-driven developmental steps is spatiotemporally associated with a different major regulatory event at the fast myosin heavy chain gene cluster, suggesting that diverse muscle gene families respond to common, or tightly integrated, regulatory signals during multiple steps of muscle fiber differentiation.

The calcineurin-NFAT pathway and muscle fiber-type gene expression

To test for a role of the calcineurin-NFAT (nuclear factor of activated T cells) pathway in the regulation of fiber type-specific gene expression, slow and fast muscle-specific promoters were examined in C2C12 myotubes and in slow and fast muscle in the presence of calcineurin or NFAT2 expression plasmids. Overexpression of active calcineurin in myotubes induced both fast and slow muscle-specific promoters but not non-muscle-specific reporters. Overexpression of NFAT2 in myotubes did not activate muscle-specific promoters, although it strongly activated an NFAT reporter. Thus overexpression of active calcineurin activates transcription of muscle-specific promoters in vitro but likely not via the NFAT2 transcription factor. Slow myosin light chain 2 (MLC2) and fast sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA1) reporter genes injected into rat soleus (slow) and extensor digitorum longus (EDL) (fast) muscles were not activated by coinjection of activated calcineurin or NFAT2 expression plasmids. However, an NFAT reporter was strongly activated by overexpression of NFAT2 in both muscle types. Calcineurin and NFAT protein expression and binding activity to NFAT oligonucleotides were different in slow vs. fast muscle. Taken together, these results indicate that neither calcineurin nor NFAT appear to have dominant roles in the induction and/or maintenance of slow or fast fiber type in adult skeletal muscle. Furthermore, different pathways may be involved in muscle-specific gene expression in vitro vs. in vivo.

Skeletal muscle gene transfer: regeneration-associated deregulation of fast troponin I fiber type specificity

Direct gene transfer into skeletal muscle in vivo presents a convenient experimental approach for studies of adult muscle gene regulatory mechanisms, including fast vs. slow fiber type specificity. Previous studies have reported preferential expression of fast myosin heavy chain and slow myosin light chain and troponin I (TnIslow) gene constructs in muscles enriched in the appropriate fiber type. We now report a troponin I fast (TnIfast) direct gene transfer study. We injected into the mouse soleus muscle plasmid DNA or recombinant adenovirus carrying a TnIfast/ beta-galactosidase (beta-gal) reporter construct that had previously been shown to be expressed specifically in fast fibers in transgenic mice. Surprisingly, microscopic histochemical analysis 1 and 4 wk postinjection showed similar TnIfast/beta-gal expression in fast and slow fibers. A low but significant level of muscle fiber segmental regeneration was evident in muscles 1 wk postinjection, and TnIfast/beta-gal expression was preferentially targeted to regenerating fiber segments. This finding can explain why TnIfast constructs are deregulated with regard to fiber type specificity, whereas the myosin constructs previously studied are not. The involvement of regenerating fiber segments in transduction by plasmid DNA and recombinant adenoviruses injected into intact normal adult muscle is an unanticipated factor that should be taken into account in the planning and interpretation of direct gene transfer experiments.

Role of denervation in modulating IIb MHC gene expression in response to T(3) plus unloading state

Previously, we have reported that the combination of hindlimb suspension (HS) and thyroid hormone [triiodothyronine (T(3))] treatment induces the de novo expression of the fast IIb myosin heavy chain (MHC) gene in the soleus. Thus we tested the hypotheses that the induction of IIb MHC expression with HS + T(3) is prevented with denervation and that this IIb induction is regulated by transcriptional processes. Adult female rats were subjected to 2 wk of combined HS + T(3) in which one side of the lower leg was simultaneously denervated. HS + T(3) caused decreased expression of the slow type I MHC and concomitant increases in both the fast type IIx and IIb MHC isoforms in the intact soleus muscle. Denervation prevented the endogenous expression of the IIb MHC gene at both the protein and mRNA levels. Although HS + T(3) intervention was able to markedly increase the expression of the 2.6-kb IIb MHC promoter-reporter construct using direct gene transfer, this induction, however, was not inhibited by denervation. These findings collectively suggest that normal innervation is essential for inducing the unique expression of the IIb MHC in a slow muscle in response to HS + T(3); however, in the denervated muscle, there is a discordance between the regulation of the endogenous IIb MHC gene relative to the exogenous IIb MHC promoter-reporter construct.

Structural characterization of the human fast skeletal muscle troponin I gene (TNNI2)

Three troponin I genes have been identified in vertebrates that encode the isoforms expressed in adult cardiac muscle (TNNI3), slow skeletal muscle (TNNI1) and fast skeletal muscle (TNNI2), respectively. While the organization and regulation of human cardiac and slow skeletal muscle genes have been investigated in detail, the fast skeletal troponin I gene has to date only been examined in birds. Here, we describe the structure and complete sequence of the human fast skeletal muscle troponin I gene (TNNI2) and identify putative regulatory elements within both the 5' flanking region and the first intron. In particular, a region containing MEF-2, E-box, CCAC and CAGG elements was identified in intron 1 that closely resembles the fast internal regulatory element (FIRE) of the quail intronic enhancer. We have previously shown that the fast skeletal muscle troponin I gene is located at 11p15.5 and noted potential close linkage with the fast skeletal muscle troponin T gene (TNNT3). Here, we have isolated two independent human PAC genomic clones that contain either TNNI2 or TNNT3 and demonstrate by interphase FISH mapping that they are less than 100 kb apart in the genome. The results demonstrate that the human TNNI2 gene is closely related to its avian counterparts with conserved elements within both the putative promoter and first intron. Our data further confirm close physical linkage of TNNI2 and TNNI3 on 11p15.5.

Normalization of muscle plasmid uptake by Southern blot: application to SERCA1 promoter analysis

Direct injection of plasmid DNA into muscle allows the study of promoters in a physiological environment. Because of the variability of reporter gene activity, attempts have been made to normalize activity to muscle plasmid uptake by coinjection of a second "control" plasmid whose reporter gene is constitutively expressed by a viral promoter. The purpose of this study was to evaluate the use of a control plasmid vs. Southern blot to normalize for differences in uptake of plasmids containing promoter fragments of the skeletal muscle-specific sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA1) gene. Results showed that the correlation of luciferase activity from control vs. SERCA1 plasmids is poor and that normalization by a virally driven control plasmid increased variability of SERCA1 luciferase activity. In several cases, the presence of a control plasmid inhibited SERCA1 reporter expression. When Southern blot analysis was used to normalize for differences in plasmid uptake there was less variability than with coinjection, and correlations between plasmid uptake and SERCA1 luciferase activity were better. Moreover, there were no inhibitory effects of a control plasmid allowing for optimization of injection conditions of the SERCA1 deletion constructs. The use of Southern analysis is suggested to determine whether plasmid uptake is differentially affected by physiological stimuli, muscle types, or plasmid sizes under study.

An E-box within the MHC IIB gene is bound by MyoD and is required for gene expression in fast muscle

The myosin heavy chain (MHC) IIB gene is selectively expressed in skeletal muscles, imparting fast contractile kinetics. Why the MHC IIB gene product is expressed in muscles like the tibialis anterior (TA) and not expressed in muscles like the soleus is currently unclear. It is shown here that the mutation of an E-box within the MHC IIB promoter decreased reporter gene activity in the fast-twitch TA muscle 90-fold as compared with the wild-type promoter. Reporter gene expression within the TA required this E-box for activation of a heterologous construct containing upstream regulatory regions of the MHC IIB promoter linked to the basal 70-kDa heat shock protein TATA promoter. Electrophoretic mobility shift assays demonstrated that mutation of the E-box prevented the binding of both MyoD and myogenin to this element. In cotransfected C2C12 myotubes and Hep G2 cells, MyoD preferentially activated the MHC IIB promoter in an E-box-dependent manner, whereas myogenin activated the MHC IIB promoter to a lesser extent, and in an E-box-independent manner. A time course analysis of hindlimb suspension demonstrated that the unweighted soleus muscle activated expression of MyoD mRNA before the de novo expression of MHC IIB mRNA. These data suggest a possible causative role for MyoD in the observed upregulation of MHC IIB in the unweighted soleus muscle.

The CACC box and myocyte enhancer factor-2 sites within the myosin light chain 2 slow promoter cooperate in regulating nerve-specific transcription in skeletal muscle

Previous experiments showed that activity of the -800-base pair MLC2slow promoter was 75-fold higher in the innervated soleus (SOL) compared with the noninnervated SOL muscles. Using in vivo DNA injection of MLC2slow promoter-luciferase constructs, the aim of this project was to identify regulatory sites and potential transcription factors important for slow nerve-dependent gene expression. Three sites within the proximal promoter (myocyte enhancer factor-2 (MEF2), E-box, and CACC box) were individually mutated, and the effect on luciferase expression was determined. There was no change in luciferase expression in the SOL and extensor digitorum longus (EDL) muscles when the E-box was mutated. In contrast, the MEF2 mutation resulted in a 30-fold decrease in expression in the innervated SOL muscles (10.3 versus 0.36 normalized relative light units (RLUs)). Transactivation of the MLC2slow promoter by overexpressing MEF2 was only seen in the innervated SOL (676,340 versus 2,225,957 RLUs; p < 0.01) with no effect in noninnervated SOL or EDL muscles. These findings suggest that the active MLC2slow promoter is sensitive to MEF2 levels, but MEF2 levels alone do not determine nerve-dependent expression. Mutation of the CACC box resulted in a significant up-regulation in the EDL muscles (0.23 versus 4.08 normalized RLUs). With the CACC box mutated, overexpression of MEF2 was sufficient to transactivate the MLC2slow promoter in noninnervated SOL muscles (27,536 versus 1, 605,797 RLUs). Results from electrophoretic mobility shift and supershift assays confirm MEF2 protein binding to the MEF2 site and demonstrate specific binding to the CACC sequence. These results suggest a model for nerve-dependent regulation of the MLC2slow promoter in which derepression occurs through the CACC box followed by quantitative expression through enhanced MEF2 activation.

Fiber-type-specific transcription of the troponin I slow gene is regulated by multiple elements

The regulatory elements that restrict transcription of genes encoding contractile proteins specifically to either slow- or fast-twitch skeletal muscles are unknown. As an initial step towards understanding the mechanisms that generate muscle diversity during development, we have identified a 128-bp troponin I slow upstream element (SURE) and a 144-bp troponin I fast intronic element (FIRE) that confer fiber type specificity in transgenic mice (M. Nakayama et al., Mol. Cell. Biol. 16:2408-2417, 1996). SURE and FIRE have maintained the spatial organization of four conserved motifs (3' to 5'): an E box, an AT-rich site (A/T2) that binds MEF-2, a CACC site, and a novel CAGG motif. Troponin I slow (TnIs) constructs harboring mutations in these motifs were analyzed in transiently and stably transfected Sol8 myocytes and in transgenic mice to assess their function. Mutations of the E-box, A/T2, and CAGG motifs completely abolish transcription from the TnI SURE. In contrast, mutation of the CACC motif had no significant effect in transfected myocytes or on the slow-specific transcription of the TnI SURE in transgenic mice. To assess the role of E boxes in fiber type specificity, a chimeric enhancer was constructed in which the E box of SURE was replaced with the E box from FIRE. This TnI E box chimera, which lacks the SURE NFAT site, confers essentially the same levels of transcription in transgenic mice as those conferred by wild-type SURE and is specifically expressed in slow-twitch muscles, indicating that the E box on its own cannot determine the fiber-type-specific expression of the TnI promoter. The importance of the 5' half of SURE, which bears little homology to the TnI FIRE, in muscle-specific expression was analyzed by deletion and linker scanning analyses. Removal of the 5' half of SURE (-846 to -811) results in the loss of expression in stably transfected but not in transiently expressing myocytes. Linker scanning mutations identified sequences in this region that are necessary for the function of SURE when integrated into chromatin. One of these sites (GTTAATCCG), which is highly homologous to a bicoid consensus site, binds to nuclear proteins from several mesodermal cells. These results show that multiple elements are involved in the muscle-specific activity of the TnIs promoter and that interactions between upstream and downstream regions of SURE are important for transcription in the context of native chromatin.

PGAM-M expression is regulated pretranslationally in hindlimb muscles and under altered loading conditions

Enzymatic activity from the muscle-specific isoform of phosphoglycerate mutase (PGAM-M) is higher within glycolytic skeletal muscles than in oxidative muscles. The hypothesis that PGAM-M is regulated pretranslationally among muscles of the hindlimb was tested using enzymatic assays, Western blots, and Northern blots. We further investigated the regulatory level(s) at which PGAM-M gene expression is controlled during hindlimb unweighting. PGAM-M mRNA and immunoreactive protein levels were fourfold lower in the rat soleus muscle than in the tibialis anterior (TA), plantaris, and extensor digitorum longus muscles. Four weeks of unweighting induced a 2.5-fold increase in PGAM enzymatic activity within the soleus muscle, a 1.8-fold increase in PGAM-M immunoreactivity, and a 3. 5-fold increase in PGAM-M mRNA. To examine potential transcriptional regulatory mechanisms, the proximal 400 bp of the rat PGAM-M promoter were linked to a firefly luciferase and injected into normal and unweighted TA and soleus muscles. Firefly luciferase activity was elevated two- to threefold in the TA and the unweighted soleus over the normal soleus muscle. These data suggest that PGAM-M expression is pretranslationally regulated among muscle types and within unweighted slow-twitch muscle. Furthermore, the proximal 400 bp of the PGAM-M promoter contains cis-acting sequences to allow muscle-type-specific expression of a reporter gene and responsiveness to soleus muscle unweighting.

Identification of a novel slow-muscle-fiber enhancer binding protein, MusTRD1

The molecular mechanisms which are responsible for restricting skeletal muscle gene expression to specific fiber types, either slow or fast twitch, are unknown. As a first step toward defining the components which direct slow-fiber-specific gene expression, we identified the sequence elements of the human troponin I slow upstream enhancer (USE) that bind muscle nuclear proteins. These include an E-box, a MEF2 element, and two other elements, USE B1 and USE C1. In vivo analysis of a mutation that disrupts USE B1 binding activity suggested that the USE B1 element is essential for high-level expression in slow-twitch muscles. This mutation does not, however, abolish slow-fiber specificity. A similar analysis indicated that the USE C1 element may play only a minor role. We report the cloning of a novel human USE B1 binding protein, MusTRD1 (muscle TFII-I repeat domain-containing protein 1), which is expressed predominantly in skeletal muscle. Significantly, MusTRD1 contains two repeat domains which show remarkable homology to the six repeat domains of the recently cloned transcription factor TFII-I. Furthermore, both TFII-I and MusTRD1 bind to similar but distinct sequences, which happen to conform with the initiator (Inr) consensus sequence. Given the roles of MEF2 and basic helix-loop-helix (bHLH) proteins in muscle gene expression, the similarity of TFII-I and MusTRD1 is intriguing, as TFII-I is believed to coordinate the interaction of MADS-box proteins, bHLH proteins, and the general transcription machinery.

Transcription occurs in pulses in muscle fibers

We report a novel mechanism of gene regulation in skeletal muscle fibers. Within an individual myofiber nucleus, not all muscle loci are transcriptionally active at a given time and loci are regulated independently. This phenomenon is particularly remarkable because the nuclei within a myofiber share a common cytoplasm. Both endogenous muscle-specific and housekeeping genes and transgenes are regulated in this manner. Therefore, despite the uniform protein composition of the contractile apparatus along the length of the fiber, the loci that encode this structure are not transcribed continuously. The total number of active loci for a particular gene is dynamic, changing during fetal development, regeneration, and in the adult, and potentially reflects the growth status of the fiber. The data reveal that transcription in particular stages of muscle fiber maturation occurs in pulses and is defined by a stochastic mechanism.

Cytochrome c promoter activity in soleus and white vastus lateralis muscles in rats

Cytochrome c protein and mRNA are 300 and 100% higher, respectively, in the soleus muscle (predominantly slow-twitch oxidative) than the white vastus lateralis (predominately fast-twitch glycolytic) muscle (W. W. Winder, K. M. Baldwin, and J. O. Holloszy. Eur. J. Biochem. 47: 461-467, 1974; M. M. Lai and F. W. Booth. J. Appl. Physiol. 69: 843-848, 1990). However, the mechanisms controlling these differences in cytochrome c mRNA are largely unknown. The present study employed direct plasmid injection techniques to determine whether the proximal promoter (-726 to +610) of the rat somatic cytochrome c gene was more active in the soleus than in white vastus lateralis muscles in rats. No difference between the soleus and white vastus lateralis muscles for the activities of the -726, -631, -489, -326, -215, -159 and -149 cytochrome c promoters was noted. The results of this study suggest that additional elements (outside of -726 to +610) in the cytochrome c gene may be required, or posttranscriptional regulation may account, for the higher cytochrome c mRNA in the slow-twitch oxidative muscle.

Molecular and cellular adaptation of muscle in response to physical training

Molecular biology tools can be used to answer questions as to how adaptations occur in skeletal muscle with training that could provide new frameworks to improve physical performance. A number of mRNAs for transfer of metabolic substrates into muscle cells increase after a single bout of exercise demonstrating the responsiveness of some gene expression to exercise. In stretch-induced hypertrophy SRE1 of the skeletal alpha-actin promoter is required to transactivate the promoter. Less retardation of SRF in crude nuclear extracts from the stretched muscle implies a conformational change in SRF because of the stretch. Transgenic animals will provide a tool to test questions concerned with how exercise signals adaptive changes in gene expression. Molecular biological approaches will be able to evaluate the interaction between physical activity levels and the expression of genes that modulate the susceptibility to many chronic diseases. Benefits of exercise extend beyond fitness to better health. Molecular biology is an important tool which should lead to improved physical performance and health in both elite athletes and the general public.

Nerve-responsive troponin I slow promoter does not respond to unloading

We examined the regulation of the troponin I slow (TnIs) promoter during skeletal muscle unloading-induced protein isoform transition, by using a transgenic mouse line harboring the -4,200 to +12 base pairs region of the human TnIs promoter. Eighteen female transgenic mice ( approximately 30 g body mass) were randomly divided into two groups: weight-bearing (WB) controls (n = 9) and hindlimb unloaded (HU; n = 9). The HU mice were tail suspended for 7 days. Body mass was unchanged in the WB group but was reduced (-6%; P < 0.05) after the HU treatment. Absolute soleus muscle mass (-25%) and soleus mass relative to body mass (-16%) were both lower (P < 0.05) in the HU group compared with the WB mice. Northern blot analyses indicate that 7 days of HU result in a 64% decrease (P < 0.05) in the abundance of endogenous TnIs mRNA (microg/mg muscle) in the mouse soleus. Furthermore, there is a trend for the abundance of the fast troponin I mRNA to be increased (+34%). Analysis of transgenic chloramphenicol acetyltransferase activity in the soleus muscle revealed no difference (P > 0.05) between WB and HU groups. We conclude that additional elements are necessary for the TnIs gene to respond to an unloading-induced, slow-to-fast isoform transition stimulus.

In vivo analysis of the myosin heavy chain IIB promoter region

The myosin heavy chain (MHC) IIB gene is preferentially expressed in fast-twitch muscles of the hindlimb, such as the tibialis anterior (TA). The molecular mechanism(s) for this preferential expression are unknown. The goals of the current study were 1) to determine whether the cloned region of the MHC IIB promoter contains the necessary cis-acting element(s) to drive fiber-type-specific expression of this gene in vivo, 2) to determine which region within the promoter is responsible for fiber-type-specific expression, and 3) to determine whether transcription off of the cloned region of the MHC IIB promoter accurately mimics endogenous gene expression in a muscle undergoing a fiber-type transition. To accomplish these goals, a 2.6-kilobase fragment of the promoter-enhancer region of the MHC IIB gene was cloned upstream of the firefly luciferase reporter gene and coinjected with pRL-cytomegalovirus (CMV) (CMV promoter driving the renilla luciferase reporter) into the TA and the slow soleus muscle. Firefly luciferase activity relative to renilla luciferase activity within the TA was 35-fold greater than within the soleus. Deletional analysis demonstrated that only the proximal 295 base pairs (pGL3IIB0.3) were required to maintain this muscle-fiber-type specificity. Reporter gene expression of pGL3IIB0.3 construct was significantly upregulated twofold in unweighted soleus muscles compared with normal soleus muscles. Thus the region within the proximal 295 base pairs of the MHC IIB gene contains at least one element that can drive fiber-type-specific expression of a reporter gene.

A binding site for nuclear receptors is required for the differential expression of the aldolase A fast-twitch muscle promoter in body and head muscles

In hind limb muscles, the aldolase A muscle-specific promoter is specifically expressed in glycolytic fast-twitch fibers. Here, we show that in addition, it is expressed at higher levels in trunk and limb muscles than in neck and head muscles independent of their fiber-type content. We have identified by analysis of transgenic mice a DNA element that is required for this differential expression and, to a lesser extent, for fiber-type specificity. We show that members of the nuclear receptor superfamily bind this element in skeletal muscle nuclear extracts. Interestingly, in gel mobility shift assays, different complexes were formed with this sequence in tongue nuclear extracts compared with limb or trunk muscle nuclear extracts. Therefore, binding of distinct nuclear receptors to a single regulatory sequence appears to be associated with the location-dependent expression of the aldolase A muscle-specific promoter.

Fibre type-specific and nerve-dependent regulation of myosin light chain 1 slow promoter in regenerating muscle

The regulation of a slow muscle gene, the myosin light chain 1 slow/ventricular gene, has been studied by in vivo transfection into regenerating rat skeletal muscle. Constructs containing portions of the myosin light chain 1 slow/ventricular promoter linked to reporter genes were injected into fast and slow muscles 3 days after muscle injury by bupivacaine injection, and reporter gene activity was analysed after 10 days. We report that a sequence in the 5' flanking region of the myosin light chain 1 slow/ventricular gene is able to direct slow muscle-specific regulation of reporter genes, and that the expression of the transgene, like that of the corresponding endogenous gene, is dependent on intact nerve. This study validates the use of regenerating muscle as a model for studying muscle gene regulation and is the first demonstration of a myosin gene promoter regulated by nerve activity.

Progress in transcriptionally targeted and regulatable vectors for genetic therapy

Safety is an important consideration in the development of genetic therapy protocols; for example, proteins that are therapeutic in the context of one tissue may be harmful in another. This is particularly relevant to suicide gene strategies for cancer, which require in vivo delivery of DNA and which, in general, demand that the therapeutic product be limited as far as possible to malignant cells. This has led to a requirement for "transcriptionally targeted" vectors that can restrict the expression of the therapeutic sequence to appropriate cells. Furthermore, there may be a therapeutic window for certain proteins such that levels of expression below and above certain thresholds may be ineffective or toxic, respectively. Therefore, it would also be desirable to create vectors that allow exogenous control of expression, so that levels of the therapeutic protein can be raised or lowered according to therapeutic need. In the context of transcriptional targeting, one may sometimes use cis-acting sequences to limit transgene expression to the target cell type. In genetic therapy for cancer, for example, it may be possible to identify and use transcriptional control elements that drive expression of proteins unique to, or over-expressed in, malignant cells. These controls would greatly reduce collateral expression of the transgene, and hence reduce toxicity to healthy cells. With regard to exogenous control of expression subsequent to transduction, several synthetic gene regulation systems have now been produced. In these systems, an inducer or repressor acts on a synthetic transcription factor that recognizes motifs unique to the promoter of the transgene; this allows regulated expression of the therapeutic protein without nonspecific effects on cellular promoters. It is likely that a vector will soon be produced in which tissue-restricted expression of the synthetic transcription factor is combined with regulatable transgene expression thereby allowing precise control of therapeutic protein production in specific tissues via administration of an inducing or repressing agent.

A combination of MEF3 and NFI proteins activates transcription in a subset of fast-twitch muscles

The human aldolase A pM promoter is active in fast-twitch muscles. To understand the role of the different transcription factors which bind to this promoter and determine which ones are responsible for its restricted pattern of expression, we analyzed several transgenic lines harboring different combinations of pM regulatory elements. We show that muscle-specific expression can be achieved without any binding sites for the myogenic factors MyoD and MEF2 and that a 64-bp fragment comprising a MEF3 motif and an NFI binding site is sufficient to drive reporter gene expression in some but, interestingly, not all fast-twitch muscles. A result related to this pattern of expression is that some isoforms of NFI proteins accumulate differentially in fast- and slow-twitch muscles and in distinct fast-twitch muscles. We propose that these isoforms of NFI proteins might provide a molecular basis for skeletal muscle diversity.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

Distinct regulatory elements control muscle-specific, fiber-type-selective, and axially graded expression of a myosin light-chain gene in transgenic mice

The fast alkali myosin light chain 1f/3f (MLC1f/3f) gene is developmentally regulated, muscle specific, and preferentially expressed in fast-twitch fibers. A transgene containing an MLC1f promoter plus a downstream enhancer replicates this pattern of expression in transgenic mice. Unexpectedly, this transgene is also expressed in a striking (approximately 100-fold) rostrocaudal gradient in axial muscles (reviewed by J. R. Sanes, M. J. Donoghue, M. C. Wallace, and J. P. Merlie, Cold Spring Harbor Symp. Quant. Biol. 57:451-460, 1992). Here, we analyzed the expression of mutated transgenes to map sites necessary for muscle-specific, fiber-type-selective, and axially graded expression. We show that two E boxes (myogenic factor binding sites), a homeodomain (hox) protein binding site, and an MEF2 site, which are clustered in an approximately 170-bp core enhancer, are all necessary for maximal transgene activity in muscle but not for fiber-type- or position-dependent expression. A distinct region within the core enhancer promotes selective expression of the transgene in fast-twitch muscles. Sequences that flank the core enhancer are also necessary for high-level activity in transgenic mice but have little influence on activity in transfected cells, suggesting the presence of regions resembling matrix attachment sites. Truncations of the MLC1f promoter affected position-dependent expression of the transgene, revealing distinct regions that repress transgene activity in neck muscles and promote differential expression among intercostal muscles. Thus, the whole-body gradient of expression displayed by the complete transgene may reflect the integrated activities of discrete elements that regulate expression in subsets of muscles. Finally, we show that transgene activity is not significantly affected by deletion or overexpression of the myoD gene, suggesting that intermuscular differences in myogenic factor levels do not affect patterns of transgene expression. Together, our results provide evidence for at least nine distinct sites that exert major effects on the levels and patterns of MLC1f expression in adult muscles.

Common core sequences are found in skeletal muscle slow- and fast-fiber-type-specific regulatory elements

The molecular mechanisms generating muscle diversity during development are unknown. The phenotypic properties of slow- and fast-twitch myofibers are determined by the selective transcription of genes coding for contractile proteins and metabolic enzymes in these muscles, properties that fail to develop in cultured muscle. Using transgenic mice, we have identified regulatory elements in the evolutionarily related troponin slow (TnIs) and fast (TnIf) genes that confer specific transcription in either slow or fast muscles. Analysis of serial deletions of the rat TnIs upstream region revealed that sequences between kb -0.95 and -0.5 are necessary to confer slow-fiber-specific transcription; the -0.5-kb fragment containing the basal promoter was inactive in five transgenic mouse lines tested. We identified a 128-bp regulatory element residing at kb -0.8 that, when linked to the -0.5-kb TnIs promoter, specifically confers transcription to slow-twitch muscles. To identify sequences directing fast-fiber-specific transcription, we generated transgenic mice harboring a construct containing the TnIs kb -0.5 promoter fused to a 144-bp enhancer derived from the quail TnIf gene. Mice harboring the TnIf/TnIs chimera construct expressed the transgene in fast but not in slow muscles, indicating that these regulatory elements are sufficient to confer fiber-type-specific transcription. Alignment of rat TnIs and quail TnIf regulatory sequences indicates that there is a conserved spatial organization of core elements, namely, an E box, a CCAC box, a MEF-2-like sequence, and a previously uncharacterized motif. The core elements were shown to bind their cognate factors by electrophoretic mobility shift assays, and their mutation demonstrated that the TnIs CCAC and E boxes are necessary for transgene expression. Our results suggest that the interaction of closely related transcriptional protein-DNA complexes is utilized to specify fiber type diversity.

Modular elements of the MLC 1f/3f locus confer fiber-specific transcription regulation in transgenic mice

The two proteins encoded by the fast alkali myosin light chain (MLC) 1f/3f locus are developmentally regulated, muscle specific, and expressed exclusively in fast-twitch fibers. Their expression is independently regulated by two separate promoters and a downstream enhancer. Previous studies showed a reporter gene directed by the rat MLC If promoter and MLC enhancer to exhibit correct skeletal muscle-specific expression in transgenic mice during development and to be preferentially expressed in fast-twitch Type IIB fibers [Donoghue et al., (1991) J. Cell B.ol. 115:423-434]. The MLC 3f promoter also directed muscle-specific expression of a CAT reporter gene in adult transgenic mice and showed little dependence upon the enhancer. Here, we show that the MLC 3f promoter also directs transgene expression in the fast-twitch fibers of adult skeletal muscle, but almost exclusively to fiber Types IIA and IIX. MLC 3f transgene expression occurs in only a subset of the fiber types that express the endogenous locus, indicating modular elements included in the transgene confer fiber-specific transcription regulation. MyoD protein was also found to be restricted to fiber Types IIA and IIX, providing evidence for its possible role in mediating fiber-specific gene expression.

Use of thapsigargin to study Ca2+ homeostasis in cardiac cells

Several reports have documented that thapsigargin is a potent inhibitor of the SR Ca2+ ATPase isolated from cardiac or skeletal muscle. We have characterized the specificity of this agent in intact rat cardiac myocytes using cells maintained in the whole cell voltage clamp configuration. We have shown that thapsigargin decreases the magnitude of the Ca2+ transient and the twitch by about 80% while it slows the decay rate for these responses. These changes were not accompanied by any alterations in sarcolemmal currents or in the trigger Ca2+ generated by the inward calcium current. Taken together these results reveal that the action of thapsigargin is restricted to the SR Ca2+ ATPase in intact cardiac myocytes. Furthermore, it is demonstrated unambiguously that SR intracellular Ca2+ stores are an absolute requirement for the development of contractile tension in rat heart myocytes. It is shown that thapsigargin is a valuable probe to examine the importance of SR pools of Ca2+ and the role of the Ca2+ ATPase in intact myocytes as well as in genetically altered heart cells.