Use of DNA injection for identification of slow nerve-dependent regions of the MLC2s gene

Local overexpression of V1a-vasopressin receptor enhances regeneration in tumor necrosis factor-induced muscle atrophy

Skeletal muscle atrophy occurs during disuse and aging, or as a consequence of chronic diseases such as cancer and diabetes. It is characterized by progressive loss of muscle tissue due to hypotrophic changes, degeneration, and an inability of the regeneration machinery to replace damaged myofibers. Tumor necrosis factor (TNF) is a proinflammatory cytokine known to mediate muscle atrophy in many chronic diseases and to inhibit skeletal muscle regeneration. In this study, we investigated the role of Arg-vasopressin-(AVP-)dependent pathways in muscles in which atrophy was induced by local overexpression of TNF. AVP is a potent myogenesis-promoting factor and is able to enhance skeletal muscle regeneration by stimulating Ca(2+)/calmodulin-dependent kinase and calcineurin signaling. We performed morphological and molecular analyses and demonstrated that local over-expression of the AVP receptor V1a enhances regeneration of atrophic muscle. By upregulating the regeneration/differentiation markers, modulating the inflammatory response, and attenuating fibrogenesis, the stimulation of AVP-dependent pathways creates a favourable environment for efficient and sustained muscle regeneration and repair even in the presence of elevated levels of TNF. This study highlights a novel in vivo role for AVP-dependent pathways, which may represent an interesting strategy to counteract muscle decline in aging or in muscular pathologies.

Differentiation and fiber type-specific activity of a muscle creatine kinase intronic enhancer

BACKGROUND

Hundreds of genes, including muscle creatine kinase (MCK), are differentially expressed in fast- and slow-twitch muscle fibers, but the fiber type-specific regulatory mechanisms are not well understood.

RESULTS

Modulatory region 1 (MR1) is a 1-kb regulatory region within MCK intron 1 that is highly active in terminally differentiating skeletal myocytes in vitro. A MCK small intronic enhancer (MCK-SIE) containing a paired E-box/myocyte enhancer factor 2 (MEF2) regulatory motif resides within MR1. The SIE's transcriptional activity equals that of the extensively characterized 206-bp MCK 5'-enhancer, but the MCK-SIE is flanked by regions that can repress its activity via the individual and combined effects of about 15 different but highly conserved 9- to 24-bp sequences. ChIP and ChIP-Seq analyses indicate that the SIE and the MCK 5'-enhancer are occupied by MyoD, myogenin and MEF2. Many other E-boxes located within or immediately adjacent to intron 1 are not occupied by MyoD or myogenin. Transgenic analysis of a 6.5-kb MCK genomic fragment containing the 5'-enhancer and proximal promoter plus the 3.2-kb intron 1, with and without MR1, indicates that MR1 is critical for MCK expression in slow- and intermediate-twitch muscle fibers (types I and IIa, respectively), but is not required for expression in fast-twitch muscle fibers (types IIb and IId).

CONCLUSIONS

In this study, we discovered that MR1 is critical for MCK expression in slow- and intermediate-twitch muscle fibers and that MR1's positive transcriptional activity depends on a paired E-box MEF2 site motif within a SIE. This is the first study to delineate the DNA controls for MCK expression in different skeletal muscle fiber types.

Tumor necrosis factor-alpha gene transfer induces cachexia and inhibits muscle regeneration

Chronic disease states are associated with elevated levels of inflammatory cytokines that have been demonstrated to lead to severe muscle wasting. A mechanistic understanding of muscle wasting is hampered by limited in vivo cytokine models which can be applied to emerging mouse mutants as they are generated. We developed a simple and novel approach to induce adult mouse skeletal muscle wasting based on direct gene transfer of an expression vector encoding the secreted form of the murine tumor necrosis factor-alpha (mTNFalpha). This procedure results in the production of elevated levels of circulating mTNFalpha followed by body weight loss, upregulation of Atrogin1, and muscle atrophy, including muscles distant from the site of gene transfer. We also found that mTNFalpha gene transfer resulted in a significant inhibition of regeneration following muscle injury. We conclude that in addition to being a potent inducer of cachexia, TNFalpha is a potent inhibitor of myogenesis in vivo.

Imaging transcription in vivo: distinct regulatory effects of fast and slow activity patterns on promoter elements from vertebrate troponin I isoform genes

Firing patterns typical of slow motor units activate genes for slow isoforms of contractile proteins, but it remains unclear if there is a distinct pathway for fast isoforms or if their expression simply occurs in the absence of slow activity. Here we first show that denervation in adult soleus and EDL muscles reverses the postnatal increase in expression of troponin I (TnI) isoforms, suggesting that high-level transcription of both genes in mature muscles is under neural control. We then use a combination of in vivo transfection, live muscle imaging and fluorescence quantification to investigate the role of patterned electrical activity in the transcriptional control of troponin I slow (TnIs) and fast (TnIf) regulatory sequences by directly stimulating denervated muscles with pattern that mimic fast and slow motor units. Rat soleus muscles were electroporated with green fluorescent protein (GFP) reporter constructs harbouring 2.7 and 2.1 kb of TnIs and TnIf regulatory sequences, respectively. One week later, electrodes were implanted and muscles stimulated for 12 days. The change in GFP fluorescence of individual muscle fibres before and after the stimulation was used as a measure for transcriptional responses to different patterns of action potentials. Our results indicate that the response of TnI promoter sequences to electrical stimulation is consistent with the regulation of the endogenous genes. The TnIf and TnIs enhancers were activated by matching fast and slow activity patterns, respectively. Removal of nerve-evoked activity by denervation, or stimulation with a mismatching pattern reduced transcriptional activity of both enhancers. These results strongly suggest that distinct signalling pathways couple both fast and slow patterns of activity to enhancers that regulate transcription from the fast and slow troponin I isoforms.

The MEF2 site is necessary for induction of the myosin light chain 2 slow promoter in overloaded regenerating plantaris muscle

Functional overload (OV) of the rat plantaris muscle results in a fast to slow change in muscle phenotype with induction of the slow contractile protein genes including myosin light chain 2 slow (MLC2s). To identify potential cis-acting DNA sites regulating MLC2s following ablation, plasmid constructs were transfected in vivo into regenerating overloaded plantaris muscles. Activity of the 270bp promoter (-270MLC2s) was increased in OV muscles at 28 days. Mutation of the MEF2 site (-270MEF2) knocked out the overload-induced activity of the promoter. Mutation of the Ebox (-270Ebox) resulted in an earlier induction with OV and mutation of the CACC site (-270CACC) resulted in increased activity in the CON PLN with OV induction detected by 21 days. These results demonstrate that the -270MLC2s promoter contains the elements necessary for expression of MLC2s in regenerating OV PLN. More importantly, mutation analysis of -270MLC2s promoter demonstrates that mechanical loading induced expression shares some common molecular mechanisms with slow nerve dependent model regulation. In these two models of physiological induction of MLC2s, the CACC site acts as a repressor region (on/off switch) and the MEF2 site acts to modulate quantitative expression.

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.

Characterization of general transcription factor 3, a transcription factor involved in slow muscle-specific gene expression

General transcription factor 3 (GTF3) binds specifically to the bicoid-like motif of the troponin I(slow) upstream enhancer. This motif is part of a sequence that restricts enhancer activity to slow muscle fibers. GTF3 contains multiple helix-loop-helix domains and an amino-terminal leucine zipper motif. Here we show that helix-loop-helix domain 4 is necessary and sufficient for binding the bicoid-like motif. Moreover, the affinity of this interaction is enhanced upon removal of amino-terminal sequences including domains 1 and 2, suggesting that an unmasking of the DNA binding surface may be a precondition for GTF3 to bind DNA in vivo. We have also investigated the interactions of six GTF3 splice variants of the mouse, three of which were identified in this study, with the troponin enhancer. The gamma-isoform lacking exon 23, and exons 26-28 that encode domain 6, interacted most avidly with the bicoid-like motif; the alpha- and beta- isoforms that include these exons fail to bind in gel retardation assays. We also show that GTF3 polypeptides associate with each other via the leucine zipper. We speculate that cells can generate a large number of GTF3 proteins with distinct DNA binding properties by alternative splicing and combinatorial association of GTF3 polypeptides.

Molecular dissection of DNA sequences and factors involved in slow muscle-specific transcription

Transcription is a major regulatory mechanism for the generation of slow- and fast-twitch myofibers. We previously identified an upstream region of the slow TnI gene (slow upstream regulatory element [SURE]) and an intronic region of the fast TnI gene (fast intronic regulatory element [FIRE]) that are sufficient to direct fiber type-specific transcription in transgenic mice. Here we demonstrate that the downstream half of TnI SURE, containing E box, NFAT, MEF-2, and CACC motifs, is sufficient to confer pan-skeletal muscle-specific expression in transgenic mice. However, upstream regions of SURE and FIRE are required for slow and fast fiber type specificity, respectively. By adding back upstream SURE sequences to the pan-muscle-specific enhancer, we delineated a 15-bp region necessary for slow muscle specificity. Using this sequence in a yeast one-hybrid screen, we isolated cDNAs for general transcription factor 3 (GTF3)/muscle TFII-I repeat domain-containing protein 1 (MusTRD1). GTF3 is a multidomain nuclear protein related to initiator element-binding transcription factor TF II-I; the genes for both proteins are deleted in persons with Williams-Beuren syndrome, who often manifest muscle weakness. Gel retardation assays revealed that full-length GTF3, as well as its carboxy-terminal half, specifically bind the bicoid-like motif of SURE (GTTAATCCG). GTF3 expression is neither muscle nor fiber type specific. Its levels are highest during a period of fetal development that coincides with the emergence of specific fiber types and transiently increases in regenerating muscles damaged by bupivacaine. We further show that transcription from TnI SURE is repressed by GTF3 when overexpressed in electroporated adult soleus muscles. These results suggest a role for GTF3 as a regulator of slow TnI expression during early stages of muscle development and suggest how it could contribute to Williams-Beuren syndrome.

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.

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.

Unloading induces transcriptional activation of the sarco(endo)plasmic reticulum Ca2+-ATPase 1 gene in muscle

Previous work showed that protein and mRNA levels of the "fast" isoform of the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA1) are markedly increased in unloaded slow-twitch soleus muscles, suggesting pretranslational control of gene expression [L. M. Schulte, J. Navarro, and S. C. Kandarian. Am. J. Physiol. 264 (Cell Physiol. 33): C1308-C1315, 1993]. However, because of the difficulty of measuring transcription rates from whole muscle, transcriptional activation of the SERCA1 gene with unloading has not been confirmed. Because SERCA1 pre-mRNA levels can reflect transcriptional activity, in the present study SERCA1 introns were sequenced to allow intron-directed RT-PCR measurement of SERCA1 pre-mRNA. These data were then compared with changes in SERCA1 mRNA expression in control and unloaded soleus muscles. After 2, 4, and 10 days of unloading, SERCA1 pre-mRNA and mRNA transcript levels increased significantly by two-, three-, and sevenfold, respectively (P < 0.01). Parallel increases in SERCA1 pre-mRNA and mRNA suggest transcriptional activation of the endogenous SERCA1 gene by muscle unloading. SERCA2, the cardiac/slow-twitch skeletal muscle isoform, was not markedly increased by unloading, and RNase protection assays showed no change in alternative splicing of SERCA1 or SERCA2 primary transcripts. With use of in vivo plasmid injection, the activity of a reporter gene driven by 3.6 kb of the SERCA1 5'-flanking region increased fivefold in 7-day-unloaded soleus muscles. Comparison of the magnitude of transcriptional activation of endogenous and constructed SERCA1 genes by unloading confirms the fidelity of using intronic RT-PCR to examine muscle gene transcription rates and suggests that cis-acting elements sufficient for regulating unloading-induced transcriptional activation are contained in this promoter construct.

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.

Gene regulation by patterned electrical activity during neural and skeletal muscle development

Patterned neural activity modifies central synapses during development and the physiological properties of skeletal muscle by selectively repressing or stimulating transcription of distinct genes. The effects of neural activity are mostly mediated by calcium. Of particular interest are the cellular mechanisms that may be used to sense and convert changes in calcium into specific alterations in gene expression. Recent studies have addressed the importance of spatial heterogeneity or of temporal changes in calcium levels for the regulation of gene expression.