Induction of beta-MHC transgene in overloaded skeletal muscle is not eliminated by mutation of conserved elements

Overexpression of TEAD-1 in transgenic mouse striated muscles produces a slower skeletal muscle contractile phenotype

TEA domain (TEAD) transcription factors serve important functional roles during embryonic development and in striated muscle gene expression. Our previous work has implicated a role for TEAD-1 in the fast-to-slow fiber-type transition in response to mechanical overload. To investigate whether TEAD-1 is a modulator of slow muscle gene expression in vivo, we developed transgenic mice expressing hemagglutinin (HA)-tagged TEAD-1 under the control of the muscle creatine kinase promoter. We show that striated muscle-restricted HA-TEAD-1 expression induced a transition toward a slow muscle contractile protein phenotype, slower shortening velocity (Vmax), and longer contraction and relaxation times in adult fast twitch extensor digitalis longus muscle. Notably, HA-TEAD-1 overexpression resulted in an unexpected activation of GSK-3alpha/beta and decreased nuclear beta-catenin and NFATc1/c3 protein. These effects could be reversed in vivo by mechanical overload, which decreased muscle creatine kinase-driven TEAD-1 transgene expression, and in cultured satellite cells by TEAD-1-specific small interfering RNA. These novel in vivo data support a role for TEAD-1 in modulating slow muscle gene expression.

MCAT elements and the TEF-1 family of transcription factors in muscle development and disease

MCAT elements are located in the promoter-enhancer regions of cardiac, smooth, and skeletal muscle-specific genes including cardiac troponin T, beta-myosin heavy chain, smooth muscle alpha-actin, and skeletal alpha-actin, and play a key role in the regulation of these genes during muscle development and disease. The binding factors of MCAT elements are members of the transcriptional enhancer factor-1 (TEF-1) family. However, it has not been fully understood how these transcription factors confer cell-specific expression in muscle, because their expression patterns are relatively broad. Results of recent studies revealed multiple mechanisms whereby TEF-1 family members control MCAT element-dependent muscle-specific gene expression, including posttranslational modifications of TEF-1 family members, the presence of muscle-selective TEF-1 cofactors, and cell-selective control of TEF-1 accessibility to MCAT elements. In addition, of particular interest, recent studies regarding MCAT element-dependent transcription of the myocardin gene and the smooth muscle alpha-actin gene in muscle provide evidence for the transcriptional diversity among distinct cell types and subtypes. This article summarizes the role of MCAT elements and the TEF-1 family of transcription factors in muscle development and disease, and reviews recent progress in our understanding of the transcriptional regulatory mechanisms involved in MCAT element-dependent muscle-specific gene expression.

Puralpha and Purbeta collaborate with Sp3 to negatively regulate beta-myosin heavy chain gene expression during skeletal muscle inactivity

Adult skeletal muscle retains the capability of transcriptional reprogramming. This attribute is readily observable in the non-weight-bearing (NWB) soleus muscle, which undergoes a slow-to-fast fiber type transition concurrent with decreased beta-myosin heavy chain (betaMyHC) gene expression. Our previous work showed that Sp3 contributes to decreased betaMyHC gene expression under NWB conditions. In this study, we demonstrate that physical and functional interactions between Sp3, Puralpha, and Purbeta proteins mediate repression of betaMyHC expression under NWB conditions. Binding of Puralpha or Purbeta to the single-stranded betaMyHC distal negative regulatory element-sense strand (dbetaNRE-S) element is markedly increased under NWB conditions. Ectopic expression of Puralpha and Purbeta decreased betaMyHC reporter gene expression, while mutation of the dbetaNRE-S element increased expression in C2C12 myotubes. The dbetaNRE-S element conferred Pur-dependent decreased expression on a minimal thymidine kinase promoter. Short interfering RNA sequences specific for Sp3 or for Puralpha and Purbeta decreased endogenous Sp3 and Pur protein levels and increased betaMyHC reporter gene expression in C2C12 myotubes. Immunoprecipitation assays revealed an association between endogenous Puralpha, Purbeta, and Sp3, while chromatin immunoprecipitation assays demonstrated Puralpha, Purbeta, and Sp3 binding to the betaMyHC proximal promoter region harboring the dbetaNRE-S and C-rich elements in vivo. These data demonstrate that Pur proteins collaborate with Sp3 to regulate a transcriptional program that enables muscle cells to remodel their phenotype.

Histochemical characteristics of soleus muscle in angiotensin-converting enzyme gene knockout mice

We examined the histochemical characteristics of soleus muscle in the angiotensin-converting enzyme (ACE) gene (Ace in mice, ACE in humans) knockout mice. Serial sections of soleus muscle of wild-type (Ace+/+, n=20) and heterozygous mutant (Ace+/-, n=24) mice were stained for myosin adenosine triphosphatase activity to identify different muscle fiber types. Capillaries were visualized by amylase-periodic acid-Schiff staining. ACE activity in the serum and gastrocnemius muscle was higher in male mice than in female mice. Female and male Ace+/- mice had markedly lower ACE activity in the serum and the gastrocnemius muscle than did female and male Ace+/+ mice, respectively. In both male and female mice, the composition of fiber types (type I and IIa) did not differ significantly between Ace+/+ and Ace+/- mice. There was no significant gender difference in capillary density. Ace+/- mice had significantly more capillaries around type IIa fibers (5.44 +/- 0.18 vs. 5.01 +/- 0.13, p<0.05) than Ace+/+ mice. The differences in the number of capillaries around type I fibers and in the number of capillaries around per fiber (capillary:fiber ratio) between Ace+/- and Ace+/+ mice were not significant (p<0.1). There was no significant difference in the mean cross-sectional area occupied by one capillary and the number of capillaries per fiber area between Ace+/+ and Ace+/- mice. In conclusion, knockout of the Ace gene in mice increased capillary density, as expressed by the mean number of capillaries around type IIa fibers. This finding suggests a possible mechanism for the cardioprotective effects of ACE inhibitors.

A novel cell-based system for evaluating skeletal muscle cell hypertrophy-inducing agents

Skeletal muscle is a tissue that adapts to increased use by increasing contractile protein gene expression and ultimately skeletal muscle mass (hypertrophy). To identify hypertrophy-inducing agents that may be potentially useful in the treatment of age-related muscle loss (sarcopenia) and to better understand hypertrophy signal transduction pathways, we have created a skeletal muscle cell-based hypertrophy-responsive system. This system was created by permanently modifying the relatively undifferentiated C2C12 cell line so that it contains the beta-myosin heavy chain (beta-MHC) gene promoter and enhancer regions fused to a luciferase reporter gene. This cell line responds, by increasing luciferase expression, to a variety of skeletal muscle hypertrophy-inducing agents, including insulin, insulin-like growth factor I, testosterone, and the beta-adrenergic receptor agonist isoproterenol, in both the undifferentiated and differentiated states. This cell-based system should be useful for identifying novel hypertrophy-inducing agents as well as understanding hypertrophy signal transduction.

Sp3 proteins negatively regulate beta myosin heavy chain gene expression during skeletal muscle inactivity

In adult skeletal muscle, beta myosin heavy chain (betaMyHC) gene expression is primarily restricted to slow type I fibers; however, its expression is down-regulated in response to muscle inactivity. Little is known about the signaling pathways and transcription factors that mediate this important functional response. This study demonstrates that increased binding of Sp3 to GC-rich elements in the betaMyHC promoter is a critical event in down-regulation of betaMyHC gene expression under non-weight-bearing conditions. Conversely, binding of Sp3 to these elements decreased while Sp1 binding increased with nuclear extracts from plantaris muscle exposed to mechanical overload, a stimulus that increases betaMyHC gene expression. In addition, these experiments revealed the existence of an Sp4-DNA binding complex when using adult skeletal muscle nuclear extract was used but not when nuclear extracts from cultured myotubes were used. Sp3 proteins are competitive inhibitors of Sp1-mediated betaMyHC reporter gene transactivation in both Drosophila SL-2 and mouse C2C12 myotubes. Sp4 is a weak activator of betaMyHC gene expression in SL-2 cells, which lack endogenous Sp1 activity, but does not activate betaMyHC gene expression in C2C12 myotubes, which have high levels of Sp1. These results suggest that competitive binding of Sp family proteins regulate betaMyHC gene transcription in response to altered neuromuscular activity.

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.

Transcription enhancer factor 1 binds multiple muscle MEF2 and A/T-rich elements during fast-to-slow skeletal muscle fiber type transitions

In adult mouse skeletal muscle, beta-myosin heavy chain (betaMyHC) gene expression is primarily restricted to slow type I fibers; however, its expression can be induced in fast type II fibers in response to a sustained increase in load-bearing work (mechanical overload [MOV]). Our previous betaMyHC transgenic and protein-DNA interaction studies have identified an A/T-rich element (betaA/T-rich -269/-258) that is required for slow muscle expression and which potentiates MOV responsiveness of a 293-bp betaMyHC promoter (beta293wt). Despite the GATA/MEF2-like homology of this element, we found binding of two unknown proteins that were antigenically distinct from GATA and MEF2 isoforms. By using the betaA/T-rich element as bait in a yeast one-hybrid screen of an MOV-plantaris cDNA library, we identified nominal transcription enhancer factor 1 (NTEF-1) as the specific betaA/T-rich binding factor. Electrophoretic mobility shift assay analysis confirmed that NTEF-1 represents the enriched binding activity obtained only when the betaA/T-rich element is reacted with MOV-plantaris nuclear extract. Moreover, we show that TEF proteins bind MEF2 elements located in the control region of a select set of muscle genes. In transient-coexpression assays using mouse C2C12 myotubes, TEF proteins transcriptionally activated a 293-bp betaMyHC promoter devoid of any muscle CAT (MCAT) sites, as well as a minimal thymidine kinase promoter-luciferase reporter gene driven by three tandem copies of the desmin MEF2 or palindromic Mt elements or four tandem betaA/T-rich elements. These novel findings suggest that in addition to exerting a regulatory effect by binding MCAT elements, TEF proteins likely contribute to regulation of skeletal, cardiac, and smooth muscle gene networks by binding select A/T-rich and MEF2 elements under basal and hypertrophic conditions.

Transcriptional regulation of the type I myosin heavy chain gene in denervated rat soleus

Denervation (DEN) of rat soleus is associated with a decreased expression of slow type I myosin heavy chain (MHC) and an increased expression of the faster MHC isoforms. The molecular mechanisms behind these shifts remain unclear. We first investigated endogenous transcriptional activity of the type I MHC gene in normal and denervated soleus muscles via pre-mRNA analysis. Our results suggest that the type I MHC gene is regulated via transcriptional processes in the denervated soleus. Deletion and mutational analysis of the rat type I MHC promoter was then used to identify cis elements or regions of the promoter involved in this response. DEN significantly decreased in vivo activity of the -3,500, -2,500, -914, -408, -299, and -215 bp type I MHC promoters, relative to the alpha-skeletal actin promoter. In contrast, normalized -171 promoter activity was unchanged. Mutation of the betae3 element (-214/-190) in the -215 promoter and deletion of this element (-171 promoter) blunted type I downregulation with DEN. In contrast, betae3 mutation in the -408 promoters was not effective in attenuating the DEN response, suggesting the existence of additional DEN-responsive sites between -408 and -215. Western blotting and gel mobility supershift assays demonstrated decreased expression and DNA binding of transcription enhancer factor 1 (TEF-1) with DEN, suggesting that this decrease may contribute to type I MHC downregulation in denervated muscle.

Divergence in species and regulatory role of beta -myosin heavy chain proximal promoter muscle-CAT elements

We examined the functional role of distinct muscle-CAT (MCAT) elements during non-weight-bearing (NWB) regulation of a wild-type 293-base pair beta-myosin heavy chain (beta MyHC) transgene. Electrophoretic mobility shift assays (EMSA) revealed decreased NTEF-1, poly(ADP-ribose) polymerase, and Max binding at the human distal MCAT element when using NWB soleus vs. control soleus nuclear extract. Compared with the wild-type transgene, expression assays revealed that distal MCAT element mutation decreased basal transgene expression, which was decreased further in response to NWB. EMSA analysis of the human proximal MCAT (pMCAT) element revealed low levels of NTEF-1 binding that did not differ between control and NWB extract, whereas the rat pMCAT element displayed robust NTEF-1 binding that decreased when using NWB soleus extracts. Differences in binding between human and rat pMCAT elements were consistent whether using rat or mouse nuclear extract or in vitro synthesized human TEF-1 proteins. Our results provide the first evidence that 1) different binding properties and likely regulatory functions are served by the human and rat pMCAT elements, and 2) previously unrecognized beta MyHC proximal promoter elements contribute to NWB regulation.

Functional overload increases beta-MHC promoter activity in rodent fast muscle via the proximal MCAT (betae3) site

Functional overload (OL) of the rat plantaris muscle by the removal of synergistic muscles induces a shift in the myosin heavy chain (MHC) isoform expression profile from the fast isoforms toward the slow type I, or, beta-MHC isoform. Different length rat beta-MHC promoters were linked to a firefly luciferase reporter gene and injected in control and OL plantaris muscles. Reporter activities of -3,500, -914, -408, and -215 bp promoters increased in response to 1 wk of OL. The smallest -171 bp promoter was not responsive to OL. Mutation analyses of putative regulatory elements within the -171 and -408 bp region were performed. The -408 bp promoters containing mutations of the betae1, distal muscle CAT (MCAT; betae2), CACC, or A/T-rich (GATA), were still responsive to OL. Only the proximal MCAT (betae3) mutation abolished the OL response. Gel mobility shift assays revealed a significantly higher level of complex formation of the betae3 probe with nuclear protein from OL plantaris compared with control plantaris. These results suggest that the betae3 site functions as a putative OL-responsive element in the rat beta-MHC gene promoter.

Transcriptional regulation of the type I myosin heavy chain promoter in inactive rat soleus

Chronic muscle inactivity with spinal cord isolation (SI) decreases expression of slow type I myosin heavy chain (MHC) while increasing expression of the faster MHC isoforms, primarily IIx. The purpose of this study was to determine whether type I MHC downregulation in the soleus muscle of SI rats is regulated transcriptionally and to identify cis-acting elements or regions of the rat type I MHC gene promoter involved in this response. One week of SI significantly decreased in vivo activity of the -3500-, -408-, -299-, -215-, and -171-bp type I MHC promoters. The activity of all tested deletions of the type I MHC promoter, relative to the human skeletal alpha-actin promoter, were significantly reduced in the SI soleus, except activity of the -171-bp promoter, which increased. Mutation of the betae3 element (-214/-190 bp) in the -215- and -408-bp promoters and deletion of this element (-171-bp promoter) attenuated type I downregulation with SI. Gel mobility shift assays demonstrated a decrease in transcription enhancer factor-1 binding to the betae3 element with SI, despite an increase in total binding to this region. These results demonstrate that type I MHC downregulation with SI is transcriptionally regulated and suggest that interactions between transcription enhancer factor-1 and the betae3 element are likely involved in this response.

Temporal effects of inactivty on myosin heavy chain gene expression in rat slow muscle

Myosin heavy chain (MHC) mRNA and protein profiles in adult rat soleus and adductor longus were determined after 4, 8, 15, 30, 60, and 90 days of spinal cord isolation (SI). SI results in complete neuromuscular inactivity while leaving the motoneuron-muscle fiber connections intact. From 15 to 90 days, type I MHC mRNA was significantly decreased, whereas type I MHC protein did not significantly decrease until 30 and 60 days in the soleus and adductor longus, respectively. However, in both muscles, slow MHC downregulation was offset by significant upregulation of the faster MHC isoforms, primarily IIx. From 60 to 90 days, type I MHC was almost completely replaced with faster isoforms at the mRNA and protein levels. Thus, chronic inactivity and unloading of slow rat hindlimb muscles shifted the MHC profile from predominately type I to type IIx MHC mRNA and protein.

Multiprotein complex formation at the beta myosin heavy chain distal muscle CAT element correlates with slow muscle expression but not mechanical overload responsiveness

To examine the role of the beta-myosin heavy chain (betaMyHC) distal muscle CAT (MCAT) element in muscle fiber type-specific expression and mechanical overload (MOV) responsiveness, we conducted transgenic and in vitro experiments. In adult transgenic mice, mutation of the distal MCAT element led to significant reductions in chloramphenicol acetyltransferase (CAT) specific activity measured in control soleus and plantaris muscles when compared with wild type transgene beta293WT but did not abolish MOV-induced CAT specific activity. Electrophoretic mobility shift assay revealed the formation of a specific low migrating nuclear protein complex (LMC) at the betaMyHC MCAT element that was highly enriched only when using either MOV plantaris or control soleus nuclear extract. Scanning mutagenesis of the betaMyHC distal MCAT element revealed that only the nucleotides comprising the core MCAT element were essential for LMC formation. The proteins within the LMC when using either MOV plantaris or control soleus nuclear extracts were antigenically related to nominal transcription enhancer factor 1 (NTEF-1), poly(ADP-ribose) polymerase (PARP), and Max. Only in vitro translated TEF-1 protein bound to the distal MCAT element, suggesting that this multiprotein complex is tethered to the DNA via TEF-1. Protein-protein interaction assays revealed interactions between nominal TEF-1, PARP, and Max. Our studies show that for transgene beta293 the distal MCAT element is not required for MOV responsiveness but suggest that a multiprotein complex likely comprised of nominal TEF-1, PARP, and Max forms at this element to contribute to basal slow fiber expression.

Effects of different activity and inactivity paradigms on myosin heavy chain gene expression in striated muscle

The goal of this mini-review is to summarize findings concerning the role that different models of muscular activity and inactivity play in altering gene expression of the myosin heavy chain (MHC) family of motor proteins in mammalian cardiac and skeletal muscle. This was done in the context of examining parallel findings concerning the role that thyroid hormone (T(3), 3,5,3'-triiodothyronine) plays in MHC expression. Findings show that both cardiac and skeletal muscles of experimental animals are initially undifferentiated at birth and then undergo a marked level of growth and differentiation in attaining the adult MHC phenotype in a T(3)/activity level-dependent fashion. Cardiac MHC expression in small mammals is highly sensitive to thyroid deficiency, diabetes, energy deprivation, and hypertension; each of these interventions induces upregulation of the beta-MHC isoform, which functions to economize circulatory function in the face of altered energy demand. In skeletal muscle, hyperthyroidism, as well as interventions that unload or reduce the weight-bearing activity of the muscle, causes slow to fast MHC conversions. Fast to slow conversions, however, are seen under hypothyroidism or when the muscles either become chronically overloaded or subjected to intermittent loading as occurs during resistance training and endurance exercise. The regulation of MHC gene expression by T(3) or mechanical stimuli appears to be strongly regulated by transcriptional events, based on recent findings on transgenic models and animals transfected with promoter-reporter constructs. However, the mechanisms by which T(3) and mechanical stimuli exert their control on transcriptional processes appear to be different. Additional findings show that individual skeletal muscle fibers have the genetic machinery to express simultaneously all of the adult MHCs, e.g., slow type I and fast IIa, IIx, and IIb, in unique combinations under certain experimental conditions. This degree of heterogeneity among the individual fibers would ensure a large functional diversity in performing complex movement patterns. Future studies must now focus on 1) the signaling pathways and the underlying mechanisms governing the transcriptional/translational machinery that control this marked degree of plasticity and 2) the morphological organization and functional implications of the muscle fiber's capacity to express such a diversity of motor proteins.

In vivo regulation of the beta-myosin heavy chain gene in soleus muscle of suspended and weight-bearing rats

In the weight-bearing hindlimb soleus muscle of the rat, approximately 90% of muscle fibers express the beta-myosin heavy chain (beta-MHC) isoform protein. Hindlimb suspension (HS) causes the MHC isoform population to shift from beta toward the fast MHC isoforms. Our aim was to establish a model to test the hypothesis that this shift in expression is transcriptionally regulated through specific cis elements of the beta-MHC promoter. With the use of a direct gene transfer approach, we determined the activity of different length beta-MHC promoter fragments, linked to a firefly luciferase reporter gene, in soleus muscle of control and HS rats. In weight-bearing rats, the relative luciferase activity of the longest beta-promoter fragment (-3500 bp) was threefold higher than the shorter promoter constructs, which suggests that an enhancer sequence is present in the upstream promoter region. After 1 wk of HS, the reporter activities of the -3500-, -914-, and -408-bp promoter constructs were significantly reduced ( approximately 40%), compared with the control muscles. However, using the -215-bp construct, no differences in promoter activity were observed between HS and control muscles, which indicates that the response to HS in the rodent appears to be regulated within the -408 and -215 bp of the promoter.

Myosin heavy chain isoform expression following reduced neuromuscular activity: potential regulatory mechanisms

In this review, the adaptations in myosin heavy chain (MHC) isoform expression induced by chronic reductions in neuromuscular activity (including electrical activation and load bearing) of the intact neuromuscular unit are summarized and evaluated. Several different animal models and human clinical conditions of reduced neuromuscular activity are categorized based on the manner and extent to which they alter the levels of electrical activation and load bearing, resulting in three main categories of reduced activity. These are: 1) reduced activation and load bearing (including spinal cord injury, spinal cord transection, and limb immobilization with the muscle in a shortened position); 2) reduced loading (including spaceflight, hindlimb unloading, bed rest, and unilateral limb unloading); and 3) inactivity (including spinal cord isolation and blockage of motoneuron action potential conduction by tetrodotoxin). All of the models discussed resulted in increased expression of fast MHC isoforms at the protein and/or mRNA levels in slow and fast muscles (with the possible exception of unilateral limb unloading in humans). However, the specific fast MHC isoforms that are induced (usually the MHC-IIx isoform in slow muscle and the MHC-IIb isoform in fast muscle) and the degree and rate of adaptation are dependent upon the animal species and the specific model or condition that is being studied. Recent studies designed to elucidate the mechanisms by which electrical activation and load bearing alter expression of MHC isoforms at the cellular and genetic levels are also reviewed. Two main mechanisms have been proposed, the myogenin:MyoD and calcineurin:NF-AT pathways. Collectively, the data suggest that the regulation of MHC isoform expression involves a complex interaction of multiple control mechanisms including the myogenin:MyoD and calcineurin:NF-AT pathways; however, other intracellular signaling pathways are likely to contribute.

Integrin signaling's potential for mediating gene expression in hypertrophying skeletal muscle

Overloaded skeletal muscle undergoes dramatic shifts in gene expression, which alter both the phenotype and mass. Molecular biology techniques employing both in vivo and in vitro hypertrophy models have demonstrated that mechanical forces can alter skeletal muscle gene regulation. This review's purpose is to support integrin-mediated signaling as a candidate for mechanical load-induced hypertrophy. Research quantifying components of the integrin-signaling pathway in overloaded skeletal muscle have been integrated with knowledge regarding integrins role during development and cardiac hypertrophy, with the hope of demonstrating the pathway's importance. The role of integrin signaling as an integrator of mechanical forces and growth factor signaling during hypertrophy is discussed. Specific components of integrin signaling, including focal adhesion kinase and low-molecular-weight GTPase Rho are mentioned as downstream targets of this signaling pathway. There is a need for additional mechanistic studies capable of providing a stronger linkage between integrin-mediated signaling and skeletal muscle hypertrophy; however, there appears to be abundant justification for this type of research.

Nuclear protein binding at the beta-myosin heavy chain A/T-rich element is enriched following increased skeletal muscle activity

In adult mouse skeletal muscle, beta-myosin heavy chain (betaMyHC) gene expression is primarily restricted to slow-type I fibers but can be induced in fast-type II fibers by mechanical overload (MOV). Our previous transgenic analyses have delimited an 89-base pair (bp) MOV-responsive region (-293 to -205), and shown that mutation of the MCAT and C-rich elements within this region did not abolish betaMyHC transgene induction by MOV. In this study we describe an A/T-rich element (betaA/T-rich; -269 5'-GGAGATATTTTT-3' -258) located within this 89-bp region that, only under MOV conditions, revealed enriched binding as characterized by electrophoretic mobility shift assays and dimethyl sulfate and diethyl pyrocarbonate interference footprinting. Direct, competition, and supershift electrophoretic mobility shift assays revealed highly enriched specific binding activity at the betaA/T-rich element that was antigenically distinct from GATA-4, MEF2A-D, SRF, and Oct-1, nuclear proteins that were previously shown to bind A/T-rich elements. In vitro translated GATA-4, MEF2C, SRF, and Oct-1 bound to consensus GATA, MEF2, SRE, and Oct-1 elements, respectively, but not to the betaA/T-rich element. Two-dimensional UV cross-linking of the bromodeoxyuridine-substituted betaA/T-rich element with mechanically overloaded plantaris (MOV-P) nuclear extract detected two proteins (44 and 48 kDa). Our results indicate that the betaA/T-rich element may function in vivo as a betaMyHC MOV-inducible element during hypertrophy of adult skeletal muscle by binding two distinct proteins identified only in MOV-P nuclear extract.

The adaptive response of MyoD family proteins in overloaded, regenerating and denervated rat muscles

Using Western blot analysis, we investigated whether the amount of myogenic regulatory factors differs in slow-type and fast-type muscles. In addition, we examined the adaptive response of myogenic regulatory factor protein in the overloaded rat muscles by the ablation of synergists, in the regenerating muscles following bupivacaine injection and in the denervated muscle. The amount of myogenin protein in the slow-type muscle was markedly greater. In contrast, the proteins MyoD and Myf-5 were selectively accumulated in the fast-type muscles. A gradual down-regulation of MyoD and Myf-5 proteins was detected in the denervated fast-type muscles, but not in the myogenin protein content. A rapid down-regulation of myogenic regulatory factor protein was observed both of the mechanically overloaded and in the regenerating muscles. These results indicate that the fast-type-specific gene expression in muscle is modulated by MyoD and Myf-5 proteins and suggest that myogenin protein plays an important role in the reconstruction of damaged neuromuscular connections.

Muscle regeneration and fiber-type transformation during distraction osteogenesis

The successful outcome of distraction osteogenesis depends in part on the adequate adaptation of the surrounding soft tissue. We characterized the adaptation of the tibialis anterior during distraction osteogenesis at different rates (0.7 and 1.4 mm/day) and amounts (15 and 30%) of lengthening. We documented the increased expression of neonatal and slow myosin heavy chain in the tibialis anterior of skeletally immature rabbits. There was neither expression of neonatal myosin heavy chain in the experimental soleus or in the slow muscle fibers of the tibialis anterior nor increased expression of slow myosin heavy chain in the soleus or gastrocnemius. The increased amount of neonatal myosin heavy chain was concentrated in the distal half of the muscle, whereas the increase in the number of fibers that were labeled with antibodies to slow myosin occurred to the same extent throughout the tibialis anterior. Electrophysiological methods showed that the tibialis anterior was functionally intact during and after distraction osteogenesis. We concluded that in the tibialis anterior of young, skeletally immature animals, distraction osteogenesis seems to induce a recapitulation of the developmental process without leading to functional changes. In addition, during distraction osteogenesis, a fiber-type transformation occurs similar to that observed in models of muscle overloading.

Segregated regulatory elements direct beta-myosin heavy chain expression in response to altered muscle activity

Our previous transgenic analyses revealed that a 600-base pair beta-myosin heavy chain (betaMyHC) promoter conferred mechanical overload (MOV) and non-weight-bearing (NWB) responsiveness to a chloramphenicol acetyltransferase reporter gene. Whether the same DNA regulatory element(s) direct betaMyHC expression following MOV or NWB activity in vivo remains unknown. We now show that a 293-base pair betaMyHC promoter fused to chloramphenicol acetyltransferase (beta293) responds to MOV, but not NWB activity, indicating a segregation of these two diverse elements. Inclusion of the betaMyHC negative regulatory element (-332 to -300; betaNRE) within transgene beta350 repressed expression in all transgenic lines. Electrophoretic mobility shift assays showed highly enriched binding activity only in NWB soleus nuclear extracts that was specific to the distal region of the betaNRE sense strand (dbetaNRE-S; -332 to -311). Supershift electrophoretic mobility shift assay revealed that the binding at the distal region of the betaNRE sense strand was antigenically distinct from cellular nucleic acid-binding protein and Y-box-binding factor 1, two proteins shown to bind this element. Two-dimensional UV cross-linking and shift Southwestern blotting analyses detected two proteins (50 and 52 kDa) that bind to this element. These in vivo results demonstrate that segregated betaMyHC promoter elements transcriptionally regulate betaMyHC transgene expression in response to two diverse modes of neuromuscular activity.

Changes in muscle mass and phenotype and the expression of autocrine and systemic growth factors by muscle in response to stretch and overload

The study of the underlying mechanisms by which cells respond to mechanical stimuli, i.e. the link between the mechanical stimulus and gene expression, represents a new and important area in the morphological sciences. Several cell types ('mechanocytes'), e.g. osteoblasts and fibroblasts as well as smooth, cardiac and skeletal muscle cells are activated by mechanical strain and there is now mounting evidence that this involves the cytoskeleton. Muscle offers one of the best opportunities for studying this type of mechanotransduction as the mechanical activity generated by and imposed upon muscle tissue can be accurately controlled and measured in both in vitro and in vivo systems. Muscle is highly responsive to changes in functional demands. Overload leads to hypertrophy, whilst decreased load force generation and immobilisation with the muscle in the shortened position leads to atrophy. For instance it has been shown that stretch is an important mechanical signal for the production of more actin and myosin filaments and the addition of new sarcomeres in series and in parallel. This is preceded by upregulation of transcription of the appropriate genes some of which such as the myosin isoforms markedly change the muscle phenotype. Indeed, the switch in the expression induced by mechanical activity of myosin heavy chain genes which encode different molecular motors is a means via which the tissue adapts to a given type of physical activity. As far as increase in mass is concerned, our group have cloned the cDNA of a splice variant of IGF-1 that is produced by active muscle that appears to be the factor that controls local tissue repair, maintenance and remodelling. From its sequence it can be seen that it is derived from the IGF-1 gene by alternative splicing but it has different exons to the liver isoforms. It has a 52 base insert in the E domain which alters the reading frame of the 3' end. Therefore, this splice variant of IGF-1 is likely to bind to a different binding protein which exists in the interstitial tissue spaces of muscle, neuronal tissue and bone. This would be expected to localise its action as it would be unstable in the unbound form which is important as its production would not disturb the glucose homeostasis unduly. This new growth factor has been called mechano growth factor (MGF) to distinguish it from the liver IGFs which have a systemic mode of action. Although the liver is usually thought of as the source of circulating IGF-1, it has recently been shown that during exercise skeletal muscle not only produces much of the circulating IGF-1 but active musculature also utilises most of the IGF-I produced. We have cloned both an autocrine and endocrine IGF-1, both of which are upregulated in cardiac as well as skeletal muscle when subjected to overload. It has been shown that, in contrast to normal muscle, MGF is not detectable in dystrophic mdx muscles even when subjected to stretch and stretch combined with electrical stimulation. This is true for muscular dystrophies that are due to the lack of dystrophin (X-linked) and due to a laminin deficiency (autosomal), thus indicating that the dystrophin cytoskeletal complex may be involved in the mechanotransduction mechanism. When this complex is defective the necessary systemic as well as autocrine IGF-1 growth factors required for local repair are not produced and the ensuing cell death results in progressive loss of muscle mass. The discovery of the locally produced IGF-1 appears to provide the link between the mechanical stimulus and the activation of gene expression.

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.

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.