Differential expression of muscle regulatory factor genes in normal and denervated adult rat hindlimb muscles

Creatine kinase transcript accumulation: effect of nerve during muscle development

To determine the role of the nerve in regulating the accumulation of cytoplasmic creatine kinase (CK) mRNAs in hindleg muscles of the developing mouse, the lumbosacral spinal cords of 14-day gestation mice (E14) were laser ablated, and the accumulation of muscle CK (MCK) and brain CK (BCK) mRNAs was evaluated just prior to birth with in situ hybridization. Numbers of molecules of each of these transcripts/ng total RNA in the soleus and extensor digitorum longus (EDL) muscles were determined with competitive PCR and compared to transcripts found in innervated crural muscles. Data suggest that: 1) the level of BCK mRNA accumulation in innervated hindlimb muscles peaks at E16.5 and remains at fetal levels until the second month postnatal, when it falls to the level found in the adult. Given that MCK transcripts meet or exceed adult levels by day 28 postnatal, the "down-regulation" of the BCK gene and the "up-regulation" of the MCK gene are not tightly coupled; 2) the developmental switch from BCK to MCK, as the dominant cytoplasmic CK mRNA, occurs in innervated and aneural leg muscles between E14 and E16.5, indicating this switch is not nerve dependent; 3) the absence of innervation has no effect on BCK mRNA accumulation. MCK transcripts/ng total RNA continue to increase in aneural muscle throughout the late fetal period, but from E16.5-E19.5 the MCK transcript levels in aneural muscles become progressively lower than in age-matched innervated muscles. Thus, the accumulation of the muscle specific cytoplasmic CK, but not BCK, transcripts is affected by the absence of innervation during the fetal period. Dev Dyn 1999;215:285-296.

What does chronic electrical stimulation teach us about muscle plasticity?

The model of chronic low-frequency stimulation for the study of muscle plasticity was developed over 30 years ago. This protocol leads to a transformation of fast, fatigable muscles toward slower, fatigue-resistant ones. It involves qualitative and quantitative changes of all elements of the muscle fiber studied so far. The multitude of stimulation-induced changes makes it possible to establish the full adaptive potential of skeletal muscle. Both functional and structural alterations are caused by orchestrated exchanges of fast protein isoforms with their slow counterparts, as well as by altered levels of expression. This remodeling of the muscle fiber encompasses the major, myofibrillar proteins, membrane-bound and soluble proteins involved in Ca2+ dynamics, and mitochondrial and cytosolic enzymes of energy metabolism. Most transitions occur in a coordinated, time-dependent manner and result from altered gene expression, including transcriptional and posttranscriptional processes. This review summarizes the advantages of chronic low-frequency stimulation for studying activity-induced changes in phenotype, and its potential for investigating regulatory mechanisms of gene expression. The potential clinical relevance or utility of the technique is also considered.

Myogenin induces a shift of enzyme activity from glycolytic to oxidative metabolism in muscles of transgenic mice

Physical training regulates muscle metabolic and contractile properties by altering gene expression. Electrical activity evoked in muscle fiber membrane during physical activity is crucial for such regulation, but the subsequent intracellular pathway is virtually unmapped. Here we investigate the ability of myogenin, a muscle-specific transcription factor strongly regulated by electrical activity, to alter muscle phenotype. Myogenin was overexpressed in transgenic mice using regulatory elements that confer strong expression confined to differentiated post-mitotic fast muscle fibers. In fast muscles from such mice, the activity levels of oxidative mitochondrial enzymes were elevated two- to threefold, whereas levels of glycolytic enzymes were reduced to levels 0.3-0.6 times those found in wild-type mice. Histochemical analysis shows widespread increases in mitochondrial components and glycogen accumulation. The changes in enzyme content were accompanied by a reduction in fiber size, such that many fibers acquired a size typical of oxidative fibers. No change in fiber type-specific myosin heavy chain isoform expression was observed. Changes in metabolic properties without changes in myosins are observed after moderate endurance training in mammals, including humans. Our data suggest that myogenin regulated by electrical activity may mediate effects of physical training on metabolic capacity in muscle.

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.

Reduced glycogen phosphorylase activity in denervated hindlimb muscles of rat is related to muscle atrophy and fibre type

Changes in the activity of muscle glycogen synthase or phosphorylase (GP) may be responsible for the deregulation of glycogen synthesis and storage which occurs in diabetes mellitus. To clarify the relationship between muscle atrophy, fibre type, insulin-stimulated glucose uptake and GP activity during insulin resistance, we used sciatic nerve severance to induce insulin resistance in rat hindlimb muscles and compared the above parameters in muscles with a range of fibre types. Changes were analysed by comparison with the contralateral hindlimb, which bears more weight due to denervation of the opposing limb, as well as the sham-operated and contralateral limb of a separate rat. Denervation caused a decrease in insulin-stimulated glucose uptake by 1 day after denervation and a decline of GP activity after 7 days in all muscles investigated. GP activity decreased by 73% in soleus, 36% in red gastrocnemius, 35% in tibialis and 13% in white gastrocnemius, which was related to the degree of muscle atrophy and inversely related to the overall GP activity in non-denervated muscles. GP activity in muscles of the contralateral limb from the denervated rat did not differ from either hindlimb of the sham-operated rat. We conclude that the fibre-type related reduction in insulin-stimulated glucose uptake of denervated muscle determines the change in its metabolism and it is this metabolic change which determines the mechanism, rate and degree of muscle atrophy, which is directly related to the decline in GP activity.

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.

mRNA levels of myogenic regulatory factors in rat slow and fast muscles regenerating from notexin-induced necrosis

The transcript levels of the myogenic regulatory factors (myoD, myf5, myogenin and MRF4) were measured by RT PCR in rat soleus (slow) and EDL (fast) muscles which were regenerating from notexin-induced necrosis. Some muscle fibers in the EDL were more resistant to the toxin, therefore the necrosis and the dominance of myoblasts were delayed for two days in EDL compared to soleus. In spite of this shift in time-course of necrosis, both types of muscle presented roughly similar, although variable, changes in the expression pattern of MRF mRNA levels. For both muscles, the myoD mRNA was upregulated on the first day after administration of the toxin, whereas concomitantly myf-5 mRNA disappeared but showed a substantial increase in later stages of regeneration. In contrast, the mRNA levels of the late MRFs myogenin and MRF4 decreased on day one only in the soleus, then increased on day three in both types of muscle. Meanwhile in EDL the level of MRF4 mRNA remained relatively normal. Four weeks after administration of the toxin the mRNA levels for each of the MRFs returned to nearly control levels. This shows that in spite of the different time course of the necrosis and regeneration, also documented by the microscopical morphology and the skeletal actin mRNA levels of the muscles, the level of MRF transcripts changed according to a quite predictable pattern; the upregulation corresponded to myoblast activation and the downregulation to the reinnervation.

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.

Early changes in muscle fiber size and gene expression in response to spinal cord transection and exercise

Muscles of spinal cord-transected rats exhibit severe atrophy and a shift toward a faster phenotype. Exercise can partially prevent these changes. The goal of this study was to investigate early events involved in regulating the muscle response to spinal transection and passive hindlimb exercise. Adult female Sprague-Dawley rats were anesthetized, and a complete spinal cord transection lesion (T10) was created in all rats except controls. Rats were killed 5 or 10 days after transection or they were exercised daily on motor-driven bicycles starting at 5 days after transection and were killed 0.5, 1, or 5 days after the first bout of exercise. Structural and biochemical features of soleus and extensor digitorum longus (EDL) muscles were studied. Atrophy was decreased in all fiber types of soleus and in type 2a and type 2x fibers of EDL after 5 days of exercise. However, exercise did not appear to affect fiber type that was altered within 5 days of spinal cord transection: fibers expressing myosin heavy chain 2x increased in soleus and EDL, and extensive coexpression of myosin heavy chain in soleus was apparent. Activation of satellite cells was observed in both muscles of transected rats regardless of exercise status, evidenced by increased accumulation of MyoD and myogenin. Increased expression was transient, except for MyoD, which remained elevated in soleus. MyoD and myogenin were detected both in myofiber and in satellite cell nuclei in both muscles, but in soleus, MyoD was preferentially expressed in satellite cell nuclei, and in EDL, MyoD was more readily detectable in myofiber nuclei, suggesting that MyoD and myogenin have different functions in different muscles. Exercise did not affect the level or localization of MyoD and myogenin expression. Similarly, Id-1 expression was transiently increased in soleus and EDL upon spinal cord transection, and no effect of exercise was observed. These results indicate that passive exercise can ameliorate muscle atrophy after spinal cord transection and that satellite cell activation may play a role in muscle plasticity in response to spinal cord transection and exercise. Finally, the mechanisms underlying maintenance of muscle mass are likely distinct from those controlling myosin heavy chain expression.

Expression of sarcoplasmic/endoplasmic reticulum Ca2+ ATPases in the rat extensor digitorum longus (EDL) muscle regenerating from notexin-induced necrosis

The level of sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) mRNAs and proteins have been assessed by RT-PCR, immunoblotting and immunocytochemistry in the rat extensor digitorum longus (EDL) muscles during regeneration from notexin-induced necrosis. As a result of the necrosis, SERCA1 and SERCA2 declined on days 1 and 3 after administration of the toxin. Thereupon the mRNA of the fast isoform SERCA1 rapidly increased between days 5 and 10 to the normal level. The mRNA level of the "housekeeping" SERCA2b isoform increased markedly during the actual necrosis at days 1 and 5, probably due to invading cells. Then the mRNA level of the neonatal SERCA1b splice variant increased first, and exceeded the level of the adult SERCA1a transcript on day 5. At later stages of regeneration the neonatal form was gradually replaced by the adult SERCA1a form, thus recapitulating similar changes known to occur during normal ontogenesis. Along with SERCA1, the levels of the slow isoform (SERCA2a) mRNA and protein increased on day 5, but the SERCA2a mRNA levels never rose above 10% of SERCA1 and after 10 days gradually declined again. In the normal and regenerated muscles, SERCA1 was expressed in 97% of the fibres which accounted for the population of fast-twitch fibres (type IIa, type IIb and probably type IIx/d). SERCA2a was present in 6% of the fibres of normal muscle (mostly in the slow-twitch type I fibres). At the end of regeneration the number of fibres expressing SERCA2a was twice as high and were found in small groups, unlike in normal EDL, but about 50% of these clustered fibres also expressed SERCA1.

Neural regulation of myosin gene expression in regenerating skeletal muscle

Excitation-transcription coupling, namely the process whereby plasma membrane depolarization leads to gene activation or inactivation, is still a black box for most muscle genes. Muscle regeneration is a useful model system to ask basic questions concerning the triggering signals and the transduction pathways involved in activity-dependent gene regulation. We report ongoing research in our laboratory concerning (1) myosin heavy chain changes in regenerating muscle in the presence and absence of the nerve, as well as changes induced by electrical stimulation, (2) identification of activity response elements in the promoter of a slow myosin light chain gene, and (3) potential approaches to define the transduction pathways induced by neural or electrical activity and implicated in muscle gene regulation.

Hypertrophy-stimulated myogenic regulatory factor mRNA increases are attenuated in fast muscle of aged quails

Myogenic regulatory factors (MRFs) are a family of skeletal muscle-specific transcription factors that regulate the expression of several muscle genes. This study was designed to determine whether MRF transcripts were increased in hypertrophy-stimulated muscle of adult quails and whether equivalent increases occurred in muscles of older quails. Slow-tonic anterior latissimus dorsi and fast-twitch patagialis muscles of adult, middle-aged, aged, and senescent quails were stretch overloaded for 6, 24, or 72 h, with contralateral muscles serving as controls. RNase protection assays showed that MRF4 and MyoD transcript levels were increased and myogenin and Myf5 transcripts were induced in stretch-overloaded muscles. However, MRF4 and MyoD increases were significantly attenuated in patagialis muscles of older quails. RT-PCR analyses of three MRF-regulated genes showed that increases in the transcription of these genes occurred with stretch overload, but the increases were less in muscles of older quails. In summary, attenuated MRF responses in muscles from aged animals may partially explain why muscles from older animals do not hypertrophy to the same extent as muscles from younger animals.

Proximal sequences of the aldolase A fast muscle-specific promoter direct nerve- and activity-dependent expression in transgenic mice

Muscle activity is known to modulate the muscle fiber phenotype. Changes in muscle activity (normal or experimentally induced) lead to modifications of the expression status of several muscle-specific genes. However, the transcription regulatory elements involved in the adaptative response are mainly unknown. The aldolase A muscle-specific promoter, pM, is expressed in adult fast twitch muscle with a preferential expression in fast glycolytic-2B fibers. Its activity is induced during postnatal muscle maturation, suggesting a role of nerve and/or muscle activity. Indeed, denervation of gastrocnemius in newborn mice prevented the activation of the promoter in this muscle, despite the nerve-independent formation of 2B fibers. Although the nerve was necessary for pM onset during development, denervating the gastrocnemius in adults had only mild effects on pM activity. By contrast, a transgene including the pM proximal regulatory sequences that are sufficient to reproduce the 2B fiber-specific expression of the endogenous promoter was shown to be highly sensitive to both neonatal and adult denervation. Transgenes containing muscle-specific pM proximal promoter elements were used to delineate the regulatory elements involved in this response to innervation and changes in the contractile activity pattern. Nerve- and activity-dependent elements could be localized in the 130-base pair-long proximal promoter region of the human aldolase A gene.

Expression of adenylyl cyclase mRNAs in the denervated and in the developing mouse skeletal muscle

Changes in the activity and in the expression of adenylyl cyclase (AC) were examined in mouse skeletal muscle after denervation and during development. Four isoforms of AC (AC2, AC6, AC7, and AC9) were detected by Northern blot analysis in gastrocnemius muscle, AC9 being the most abundant. After denervation, the levels of AC2 and AC9 mRNA decreased, whereas those of AC6 and AC7 increased. AC activity in response to several neurotransmitters was increased after denervation. During development, AC activity was high in fetus and neonate and declined in the adult; the sensitivity of AC activity to various neurotransmitters was the highest on the third postnatal day. The levels of AC6 and AC7 mRNAs were high on the third postnatal day and then decreased in adult, paralleling the decline in AC activity. All the characteristics of AC expression and activity in fetus and neonate resembled those observed in denervated adult muscle. These results indicate that changes in AC activity and AC mRNAs play an important role in the various physiopathological states of skeletal muscle, especially during muscle atrophy.

Myotrophic effects of muscle extracts obtained at different intervals after denervation

A study was made of the myotrophic effects of denervated muscle extracts on normal Wistar rat soleus muscle. Extracts obtained 1 h, 2, 4 and 7 days after sectioning of the sciatic nerve were administered intraperitoneally over five consecutive days. Soleus muscles were routinely processed for morphological and morphometrical analysis using light microscopic techniques. Quantitative differences were observed in the effects of different extracts on total muscle area, fibre cross-sectional area, mean minimum diameter and number of fibres/ area. The greatest myotrophic response was elicited by extracts obtained at 2 and 4 days; differences with respect to controls and extracts obtained at 1 day were significant (P < 0.05) for all parameters studied. Statistically significant differences (P < 0.05) were also recorded for fibre cross-sectional area and mean minimum diameter between the 2- and 4-day groups and the 7-day group. It may thus be concluded that the time elapsing between denervation and extract obtention influences the effect of the extract on normal rat muscle.

Myogenin, MyoD, and myosin expression after pharmacologically and surgically induced hypertrophy

The relationship between myogenin or MyoD expression and hypertrophy of the rat soleus produced either by clenbuterol and 3,3', 5-triiodo-L-thyronine (CT) treatment or by surgical overload was examined. Mature female rats were subjected to surgical overload of the right soleus with the left soleus serving as a control. Another group received the same surgical treatment but were administered CT. Soleus muscles were harvested 4 wk after surgical overload and weighed. Myosin heavy chain isoforms were separated by using polyacrylamide gel electrophoresis while myogenin and MyoD expression were evaluated by Northern analysis. CT and functional overload increased soleus muscle weight. CT treatment induced the appearance of the fast type IIX myosin heavy chain isoform, depressed myogenin expression, and induced MyoD expression. However, functional overload did not alter myogenin or MyoD expression in CT-treated or non-CT-treated rats. Thus pharmacologically and surgically induced hypertrophy have differing effects on myogenin and MyoD expression, because their levels were associated with changes in myosin heavy chain composition (especially type IIX) rather than changes in muscle mass.

Determination of muscle contractile properties: the importance of the nerve

Contractile phenotype of muscle fibres is strongly influenced by hormones, stretch and influences from the motor neurones, although cell lineage probably also plays a role. Motor neurones can affect muscle fibres by releasing neurotrophic substances and by evoking electrical activity in the muscle. For regulating contractile properties such as speed, strength and endurance it has been demonstrated that electrical activity is crucial, while the role of putative neurotrophic substances remains unclear. The signal to change is coded in the pattern of electrical activity. Thus, high amounts of activity lead to slow shortening velocity and myosin heavy chains, while low amounts of activity lead to a fast phenotype. For regulation of twitch duration frequency also plays a role, and for preventing atrophy in denervated muscles high frequency seems to be beneficial, particularly in fast muscles. Little is known about the excitation-adaptation pathway linking action potentials to expression of genes that are relevant for contractile properties. Muscle specific transcription factors of the helix-loop-helix family such as myoD and myogenin could be important for regulating genes related to metabolic profile and fibre size/strength, while their role in determining myosin heavy chain expression and classical fibre type is more uncertain.

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.

Role of the nerve in determining fetal skeletal muscle phenotype

To determine the role of the nerve on the establishment of myofiber diversity in skeletal muscles, the lumbosacral spinal cord of 14-day gestation mice (E14) was laser ablated, and the accumulation of the myosin alkali light chains (MLC) mRNAs in crural (hindleg) muscles was evaluated just prior to birth with in situ hybridization. Numbers of molecules of each alkali MLC/ng total RNA in the extensor digitorum longus (EDL) and soleus muscles were determined with competitive polymerase chain reaction. Transcripts for all four alkali MLCs accumulate in aneural muscles. Data suggest that: (1) the absence of the nerve to either future fast or slow muscles results in less accumulation of MLC1V transcript. Moreover, the presence of the nerve is required for the enhanced accumulation of this transcript in future slow muscles; (2) the absence of innervation of future slow, but not fast, muscles decreases the accumulation of MLC1A transcript. Since increased accumulation of MLC1A and MLC1V transcripts are found in future slow muscles at birth, the nerve is necessary for the development of the slow phenotype during myogenesis; (3) MLC1F and MLC3F transcripts do not display any preferential accumulation in future fast muscles during the fetal period. Therefore, the establishment of the differential distribution of these mRNAs, based on fiber type, is a postnatal phenomenon. The nerve is required during the fetal period to allow accumulation of MLC3F messages above a basal level in future fast as well as slow muscles; whereas, the absence of the innervation to future fast, but not slow, muscles reduces the accumulation of MLC1F. Thus, the accumulation of the various alkali MLC mRNAs shows a differential, rather than coordinate, response to the absence of the nerve, and this response may vary depending on the future fiber type of the muscles.

Cross-talk between transcriptional regulation by thyroid hormone and myogenin: new aspects of the Ca2+-dependent expression of the fast-type sarcoplasmic reticulum Ca2+-ATPase

We have previously demonstrated an interaction between the major determinants of skeletal muscle phenotype by showing that continuous contractile activity represses the thyroid hormone (3,3', 5-tri-iodothyronine; T3)-dependent transcriptional activity of fast-type sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase (SERCA1), a characteristic of the fast phenotype. Both the free cytosolic Ca2+ concentration ([Ca2+]i) and the myogenic determination factors MyoD and myogenin have been implicated as mediators of the effect of contractile activity on skeletal muscle phenotype. Using L6 cells we have shown that an increase in the steady-state [Ca2+]i above the resting level of 120 nM indeed can mimic the effect of contractile activity on T3-dependent SERCA1 expression. We now show that the repressing effect of increased [Ca2+]i on T3-dependent SERCA1 expression in L6 cells is exerted at a pre-translational level and is accompanied by increased myogenin mRNA expression. Myogenin overexpression in these cells revealed that increased expression of myogenin alone strongly decreases the T3-dependent stimulation of SERCA1 promoter activity. These results suggest a pathway for the regulation of skeletal muscle phenotype in which [Ca2+]i mediates the effect of contractile activity by regulating the expression of myogenin, which in turn interferes with transcriptional regulation by T3.

The functional matrix hypothesis revisited. 4. The epigenetic antithesis and the resolving synthesis

In two interrelated articles, the current revision of the functional matrix hypothesis extends to a reconsideration of the relative roles of genomic and of epigenetic processes and mechanisms in the regulation (control, causation) of craniofacial growth and development. The dialectical method was chosen to analyze this matter, because it explicitly provides for the fuller presentation of a genomic thesis, an epigenetic antithesis, and a resolving synthesis. The later two are presented here, where the synthesis suggests that both genomic and epigenetic factors are necessary causes, that neither alone is also a sufficient cause, and that only the two, interacting together, furnish both the necessary and sufficient cause(s) of ontogenesis. This article also provides a comprehensive bibliography that introduces the several new, and still evolving, disciplines that may provide alternative viewpoints capable of resolving this continuing controversy; repetition of the present theoretical bases for the arguments on both sides of these questions seems nonproductive. In their place, it is suggested that the group of disciplines, broadly termed Complexity, would most likely amply repay deeper consideration and application in the study of ontogenesis.

Myogenic regulatory factors during regeneration of skeletal muscle in young, adult, and old rats

Myogenic factor mRNA expression was examined during muscle regeneration after bupivacaine injection in Fischer 344/Brown Norway F1 rats aged 3, 18, and 31 mo of age (young, adult, and old, respectively). Mass of the tibialis anterior muscle in the young rats had recovered to control values by 21 days postbupivacaine injection but in adult and old rats remained 40% less than that of contralateral controls at 21 and 28 days of recovery. During muscle regeneration, myogenin mRNA was significantly increased in muscles of young, adult, and old rats 5 days after bupivacaine injection. Subsequently, myogenin mRNA levels in young rat muscle decreased to postinjection control values by day 21 but did not return to control values in 28-day regenerating muscles of adult and old rats. The expression of MyoD mRNA was also increased in muscles at day 5 of regeneration in young, adult, and old rats, decreased to control levels by day 14 in young and adult rats, and remained elevated in the old rats for 28 days. In summary, either a diminished ability to downregulate myogenin and MyoD mRNAs in regenerating muscle occurs in old rat muscles, or the continuing myogenic effort includes elevated expression of these mRNAs.

Identification in a fish species of two Id (inhibitor of DNA binding/differentiation)-related helix-loop-helix factors expressed in the slow oxidative muscle fibers

Helix-loop-helix (HLH) proteins related to the inhibitor of DNA binding/differentiation (Id) serve as general antagonists of cell differentiation. They lack a basic DNA-binding domain and are thought to function in a dominant negative manner by sequestering basic HLH (bHLH) transcription factors that are involved in cell determination and differentiation. Four Id-encoding genes have been shown in mammals, they have a distinct pattern of expression suggesting different functions for each member in different cell lineage. In this study we describe the identification and cloning of two trout cDNAs which encode helix-loop-helix proteins showing a high degree of similarity with mammalian Id family members. One cDNA encodes a trout putative Id1 protein (TId1) that is 63% identical to the human Id1 protein over the entire length and 78% identical within the HLH region. The other cDNA encodes a trout putative Id2 protein (TId2) that shows 82% identity to the human Id2 protein and only one change that is conservative over the HLH region. In the 3' untranslated region, TId2 mRNA exhibits 16 nucleotides upstream from the AATAAA site, a palindromic sequence similar to the cytoplasmic polyadenylation element (CPE) which is also present in Id2 and Id3 mRNAs from mammals and in XIdx/XIdI mRNA from Xenopus. In the fish, TId1 and TId2 are expressed in a tissue-specific manner, with slightly different patterns. During myogenesis, TId1 and TId2 are highly expressed in the myotomal musculature of fish embryos and of early alevins but are down-regulated in that of late alevins. In muscle from juveniles and adults, TId1 and TId2 transcripts are abundant in the slow oxidative fibers while they are absent in the fast glycolytic fibers. This expression pattern suggests that Id genes play a role in the regulation of muscle fiber phenotype in addition to controlling early myogenesis. On the whole, the identification of two HLH-Id encoding genes in a major taxonomic group like teleosts, suggests an early divergence of Id genes in vertebrate evolution. The observation that Id transcripts are present selectively in the slow muscle reveals that their expression is more complicated than previously appreciated.

Quantification of MyoD, myogenin, MRF4 and Id-1 by reverse-transcriptase polymerase chain reaction in rat muscles--effects of hypothyroidism and chronic low-frequency stimulation

A highly sensitive method of reverse-transcriptase polymerase chain reaction (RT-PCR) was established to quantify transcript levels of the myogenic regulatory factors MyoD, myogenin and MRF4 (muscle regulatory factor 4) and for Id-1 (inhibitor of differentiation), a putative negative regulator of myogenesis. The method was sensitive enough to detect mRNA amounts as low as 20 molecules. Measurements in 10 different skeletal muscles of the rat revealed that the amounts of the four factors differ by almost three orders of magnitude. Id-1 is expressed at lowest levels (approximately 4x10(5) molecules/microg RNA) and MRF4 at highest levels (approximately 9x10(7) molecules/microg RNA). In general, myogenin and MyoD mRNAs were inversely distributed in slow and fast muscles. A correlation seemed to exist between the levels of MyoD and myosin heavy chain (MHC) IIb, the fastest MHC isoform. However, as revealed by changes in the expression levels of these two regulatory factors under conditions of hypothyroidism and chronic low-frequency stimulation (CLFS), MyoD and myogenin did not seem to be strictly correlated with fast and slow myosins, respectively. Hypothyroidism led to pronounced depressions of MyoD, but only to small increases in myogenin mRNA in fast muscles. These changes were only slightly increased by CLFS. However, as previously shown, CLFS in combination with hypothyroidism induces in rat muscle pronounced fast to slow transitions in myosin expression [Kirschbaum, B. J., Kucher. H.-B., Termin, A., Kelly, A. M. & Pette, D. (1990) J. Biol. Chem. 265, 13974-13980]. These findings suggest that MyoD and myogenin may not be causally related to the development and maintenance of fiber-type diversities.

Distal regulatory elements control MRF4 gene expression in early and late myogenic cell populations

MRF4 is a muscle-specific transcription factor that belongs to a family of basic helix-loop-helix proteins known as the myogenic regulatory factors (MRFs). In vitro studies have shown that expression of the MRF4 gene is controlled by a proximal promoter element (-336 to +71) that binds the muscle-specific transcription factors MEF2 and myogenin to activate transcription. To examine further the regulatory elements necessary for endogenous MRF4 gene expression during development, transgenic mice were generated that contained either a proximal MRF4 promoter-LacZ reporter gene (-336 MRF4-nLacZ) or a MRF4-LacZ reporter gene containing 8.5 kb of 5' flanking sequence (-8500 MRF4-nLacZ). Characterization of individual transgenic mouse lines throughout development revealed that expression of both transgenes is restricted to skeletal muscle tissue. However, unlike previous in vitro data, the proximal promoter transgene exhibits only limited transcriptional activity at all developmental time points, whereas the -8500 MRF4-nLacZ lines fully recapitulate the later developmental expression patterns and exhibit transcription in myotomal cells during somitic differentiation. Tissue culture analysis of myogenic cells isolated from E12.5, E16.5, and adults confirmed that the -8500 MRF4-nLacZ transgene is expressed in greater than 90% of the myotubes for all myogenic populations. These results indicate that 8.5 kb of MRF4 5' flanking sequence contains all the regulatory elements necessary for late MRF4 expression and that at least some of these elements lie upstream of the -336 proximal promoter. It is also likely that distant upstream regulatory sequences control early somitic MRF4 expression. These findings, coupled with previous in vitro studies, suggest that the early and late developmental expression patterns of the MRF4 gene are controlled by distinct sets of regulatory elements.

Regionalized expression of myosin isoforms in heterotypic myotubes formed from embryonic and fetal rat myoblasts in vitro

The development of mammalian limb muscles involves the appearance and fusion of at least two separate populations of muscle precursor cells. These two populations, termed embryonic and fetal myoblasts, are first detected within the limb bud at different stages of development. We have previously demonstrated that, in the rat, each myoblast population expresses a unique pattern of myosin heavy chains (MyHCs) during differentiation in vitro (Pin and Merrifield [1993] Dev. Genet. 14:356-368). Embryonic myoblasts accumulate embryonic and slow MyHCs, whereas fetal myoblasts accumulate embryonic, neonatal, and adult fast MyHCs but not slow MyHC. To determine if the two populations can fuse with each other and whether the pattern of MyHC expression is altered in the resulting heterokaryons, embryonic and fetal myoblasts were labelled with the lipophilic dye PKH26, [3H]-thymidine, or 5-bromodeoxyuridine (BRDU) and cocultured for 24-48 hr. Our results demonstrate that fusion occurs between embryonic and fetal myoblasts in vitro. Moreover, analysis of the resulting heterokaryons revealed regionalized accumulations of MyHC around individual nuclei. Interestingly, these accumulations were typical of the default pattern of expression that individual nuclei would have normally expressed in single culture. Nuclei contributed by embryonic myoblasts were surrounded by localized accumulations of slow MyHC, whereas nuclei from fetal myoblasts were surrounded by neonatal/fast MyHC. The occurrence of such nuclear domains indicates that the myoblast-specific expression of MyHC isoforms is dictated by cis-acting factors established prior to fusion.

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.

MyoD protein is differentially accumulated in fast and slow skeletal muscle fibres and required for normal fibre type balance in rodents

MyoD is a muscle-specific transcription factor involved in commitment of cells to myogenesis. MyoD mRNA levels differ between fast and slow muscles, suggesting that MyoD may regulate aspects of fibre type. Here we show that detectable MyoD protein becomes restricted during development to the nuclei of the fastest classes of fibres in fast muscles. myoDm1 mice, in which the myoD gene has been disrupted, show subtle shifts in fibre type of fast muscles toward a slower character, suggesting that MyoD is involved in the maintenance of the fast IIB/IIX fibre type. In contrast, slow muscle shifts to a faster phenotype in myoDm1. Moreover, MD6.0-lacZ transgenic mice with the myoD promoter driving lacZ, show highest beta-galactosidase activity in the fastest fibres of fast muscles, but also express low levels in slow fibres of slow, but not fast, muscles, suggesting distinct regulation of gene expression in slow fibres of fast and slow muscles.

The recovery of long-term denervated rat muscles after Marcaine treatment and grafting

Disruption of the nerve supply results in the rapid loss of mass and contractile force in skeletal muscles. These losses are reversible to a high degree in short-term denervated muscles with grafting and nerve implantation. However, return is much poorer in long-term denervated muscles. This study examined the basis for the differences in the recovery of non-denervated and 7-month denervated rat extensor digitorum longus (EDL) muscles after grafting and nerve implantation. We found that the level of recovery is related to the ability of muscle fibers to degenerate and regenerate after grafting. Fibers within long-term denervated muscles do not degenerate and regenerate as well as those within muscles which are not denervated prior to grafting. The functional recovery of the denervated muscles is significantly improved when their fibers are induced to degenerate with the myotoxic anesthetic, Marcaine, Degeneration of these fibers is followed by massive regeneration. The finding that denervated muscles are capable of being restored to a significant level by inducing regeneration may be useful in the clinical treatment of denervated muscles.

Interaction of exercise training and clenbuterol on GLUT-4 protein in muscle of obese Zucker rats

Chronic administration of clenbuterol, a beta 2-adrenergic agonist, attenuates the exercise training-induced improvement in muscle insulin resistance of the obese Zucker rat. The present study was conducted to determine whether clenbuterol also attenuates the increase in muscle GLUT-4 protein that occurs with exercise training and whether the action of clenbuterol is related to its ability to downregulate the beta-adrenergic receptors. Female obese Zucker rats were randomly assigned to one of the following four groups: control (CON, n = 7), clenbuterol (CL, n = 8), exercise training (TR, n = 8), and clenbuterol with exercise training (CL+TR, n = 8). Rats assigned to the training groups were run on a rodent motor-driven treadmill for 6-7 wk. Rats receiving clenbuterol were intubated with 0.8 mg/kg body weight 30 min before running each day. Red quadriceps (RQ) and white quadriceps (WQ) GLUT-4 protein concentrations of TR rats were significantly greater than those of CON and CL+TR rats. The RQ GLUT-4 protein concentration of the CL+TR rats was significantly greater than that of CON rats, but this difference did not occur in the WQ. GLUT-4 protein concentrations were not different between the CON and CL rats. The patterns of RQ and WQ GLUT-4 mRNA were similar to those of their respective GLUT-4 proteins. Rats receiving daily injections of propranolol (30 mg/kg body wt), a beta-adrenergic receptor antagonist, demonstrated no increase in GLUT-4 protein in RQ or WQ after 6 wk of exercise training. These results indicate that 1) clenbuterol can attenuate the increase in muscle GLUT-4 protein associated with exercise training and 2) this effect is likely mediated by a downregulation of the beta-adrenergic receptors.

Developmental shift of myosin heavy chain mRNA expression due to neural factor(s) and muscle activity

The adult ventricular isoform of chicken myosin heavy chain (MHC-V) is transiently expressed in all skeletal muscle primordia analyzed and is completely repressed around embryonic days 10-12, when functional innervation is established. By ribonuclease protection assay, we demonstrated that denervation of the adult anterior latissimus dorsi muscle resulted in reexpression of MHC-V mRNA. In contrast, treatment of primary cultures of fetal breast or leg muscles with embryonic brain extract or conditioned media from glial or neuroblastoma cell lines, but not from a myogenic cell line or primary muscle cell cultures, led to inhibition of MHC-V expression. This inhibitory activity was abolished by heating and increased with protein concentration. The acquisition of both brain inhibitory activity and the competence of myogenic cells to downregulate MHC-V mRNA expression were age dependent. Furthermore, either paralysis of muscle in ovo by curare or contraction arrest of cultured myotubes resulted in persistent expression of MHC-V mRNA. Thus a putative soluble factor(s) of nerve origin as well as muscle activity are involved in the developmental downregulation of MHC-V expression in muscle primordia.

E-box sites and a proximal regulatory region of the muscle creatine kinase gene differentially regulate expression in diverse skeletal muscles and cardiac muscle of transgenic mice

Previous analysis of the muscle creatine kinase (MCK) gene indicated that control elements required for transcription in adult mouse muscle differed from those required in cell culture, suggesting that distinct modes of muscle gene regulation occur in vivo. To examine this further, we measured the activity of MCK transgenes containing E-box and promoter deletions in a variety of striated muscles. Simultaneous mutation of three E boxes in the 1,256-bp MCK 5' region, which abolished transcription in muscle cultures, had strikingly different effects in mice. The mutations abolished transgene expression in cardiac and tongue muscle and caused a reduction in expression in the soleus muscle (a muscle with many slow fibers) but did not affect expression in predominantly fast muscles: quadriceps, abdominals, and extensor digitorum longus. Other regulatory sequences with muscle-type-specific activities were found within the 358-bp 5'-flanking region. This proximal region conferred relatively strong expression in limb and abdominal skeletal muscles but was inactive in cardiac and tongue muscles. However, when the 206-bp 5' enhancer was ligated to the 358-bp region, high levels of tissue-specific expression were restored in all muscle types. These results indicate that E boxes and a proximal regulatory region are differentially required for maximal MCK transgene expression in different striated muscles. The overall results also imply that within skeletal muscles, the steady-state expression of the MCK gene and possibly other muscle genes depends on transcriptional mechanisms that differ between fast and slow fibers as well as between the anatomical and physiological attributes of each specific muscle.

Regulation of myosin heavy chain expression in adult rat hindlimb muscles during short-term paralysis: comparison of denervation and tetrodotoxin-induced neural inactivation

The extent to which myosin profiles within adult fast and slow muscles are altered by short-term paralysis remains equivocal. We used an array of specific antibodies to identify adult and developmental MHC isoforms within EDL and soleus muscle fibers, and show a marked multiple expression of MHCs with a general shift towards slower and more energy efficient MHC profiles after 2 weeks of denervation or TTX nerve conduction block. Paralysis also induced marked expression of an embryonic MHC within most EDL cell types, and a subtle, paralysis-sensitive, expression of alpha-cardiac MHC within specific EDL and soleus extrafusal fibers. Comparison of treatment groups also permitted assessment of the relative influence of neural activity versus trophic factors on these isoforms, and confirmed activity as a major, but not sole, regulator of MHC expression.

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

Mechanical overload leads to hypertrophy, increased type I fiber composition, and beta-myosin heavy chain (beta-MHC) induction in the fast-twitch plantaris muscle. To better understand the mechanism(s) involved in beta-MHC induction, we have examined inducible expression of transgenes carrying the simultaneous mutation of three DNA regulatory subregions [muscle CAT (MCAT), C-rich, and beta e3] in the context of either 5,600-base pair (bp; beta 5.6mut3) or 600-bp (beta 0.6mut3) beta-MHC promoter in overloaded plantaris muscles of transgenic mice. Protein extract from mechanically overloaded plantaris muscle of mice, harboring either mutant transgene beta 5.6mut3 or beta 0.6mut3, showed an unexpected 2.8- to 4.5-fold increase in chloramphenicol acetyltransferase (CAT) specific activity relative to their respective controls. Similar results were obtained with wild-type (wt) beta-MHC transgenes (beta 5.6wt, beta 0.6wt). Histochemical staining for both myofibrillar ATPase and CAT activity and CAT immunohistochemistry revealed a striking increase in type I fibers and that CAT expression was restricted to these fibers in overloaded plantaris muscle of beta 5.6mut3 transgenic mice. Our transgenic data suggest that beta-MHC transgenes, and perhaps the endogenous beta-MHC gene, are induced by mechanical overload via a mechanism(s) that does not exclusively require the MCAT, C-rich, or beta e3 subregions.

Myosin heavy chain profile of cat soleus following chronic reduced activity or inactivity

To determine the role that normal neuromuscular activity plays in maintaining the myosin heavy chain (MHC) profile of adult cat soleus muscles, the spinal cords of 4 cats were transected (ST) and 8 cats were spinal isolated (SI) for 6 months. Nine nonoperated cats served as controls. Electrophoresis demonstrated that the soleus from control cats contained 98% type I, and 2% IIa MHCs. Both ST and SI resulted in decreased type I and increased IIa MHC, as well as de novo expression of IIb MHC. Immunohistochemistry with MHC-specific antibodies demonstrated that the soleus from control cats contained 99% type I, 1% IIa, and < 1% hybrid fibers (containing both type I and II MHCs). Following ST there were 67% type I, 17% IIa, 3% IIb, and 13% hybrid fibers. After SI, 48% of the fibers were type I, 11% were IIa, 1% were IIb, 25% were hybrid, and 15% contained embryonic MHC. Thus, normal levels of neuromuscular activity appear to be necessary for maintenance of the normal adult MHC profile in some fibers. Complete inactivation results in developmental MHC isoform expression in some fibers. Therefore, the dependence of a fiber on activity as a source of MHC modulation differs substantially among fibers even in a relatively homogeneous muscle.

Two myogenic regulatory factor transcripts exhibit muscle-specific responses to disuse and passive stretch in adult rats

Levels of myogenic regulatory factor (MRF) transcripts are altered in a muscle-specific manner in response to hind limb immobilisation of adult male rats, for a 2 day period, in either a lengthened or shortened position which result in passive stretch or disuse atrophy respectively. Myogenin transcript levels were dramatically elevated in the stretched plantaris but not soleus, whereas the MRF4 transcript was significantly elevated in soleus but not plantaris. Levels of myogenin mRNA were unaffected by disuse in either muscle and MRF4 was markedly lower in plantaris in response to disuse.

Preferential binding of MyoD-E12 versus myogenin-E12 to the murine sarcoma virus enhancer in vitro

The MyoD family of transcription factors regulates muscle-specific gene expression in vertebrates. In the adult rat, MyoD mRNA accumulates predominately in fast-twitch muscle, in particular type IIb and/or IIx fibers, whereas Myogenin mRNA is restricted to slow-twitch type I muscle fibers. Transgenic mice expressing the avian v-ski oncogene from the murine sarcoma virus (MSV) promoter-enhancer display preferential hypertrophy of type IIb fast-twitch muscle apparently because of the restricted expression of the transgene. We tested the hypothesis that preferential interactions of MyoD, as a heterodimer with E12, with the MSV enhancer, which has six E-box targets for MyoD family proteins, could contribute to v-ski gene expression in IIb muscle fibers. A series of quantitative binding studies was performed using an electrophoretic mobility shift assay to test MyoD-E12 versus Myogenin-E12 binding to the MSV enhancer. Our results indicate that MyoD-E12 binds the MSV enhancer with higher affinity and higher cooperativity than Myogenin-E12. Interestingly, MyoD-E12 bound all of the individual E-boxes tested with positive cooperativity indicating DNA-mediated dimerization of the protein subunits.

Beta-MHC and SMLC1 transgene induction in overloaded skeletal muscle of transgenic mice

The hypertrophic responses of white fast-twitch muscle to mechanical overload has been investigated using transgenic mice. After 7 wk of overload, endogenous beta-myosin heavy chain (MHC) and slow myosin light chain 1 and 2 (SMLC1, SMLC2) protein were increased in the overloaded plantaris (OP) muscle compared with sham-operated control plantaris (CP)muscle. Concurrently, the levels of endogenous beta-MHC, SMLC1, SMLC2, and cardiac/slow troponin C (CTnC) mRNA transcripts were significantly increased in OP muscles, whereas skeletal troponin C (sTnC) mRNA transcript levels decreased. As an initial attempt to locate DNA sequence(s) that governs beta-MHC induction in response to mechanical overload, multiple independent transgenic lines harboring four different human beta-MHC transgenes (beta 1286, beta 988, beta 450, beta 141) were generated. Except for transgene beta 141, muscle-specific expression and induction (3- to 22-fold) in OP muscles were observed by measuring chloramphenicol acetyltransferase activity (CAT assay). Induction of a SMLC1 transgene (3920SMLC1) in OP muscles was also observed. Collectively, these in vivo data provide evidence that 1) a mechanical overload inducible element(s) is located between nucleotides -450 and +120 of the human beta-MHC transgene, 2) 3,900 bp of 5' sequence is sufficient to confer mechanical overload induction of a SMLC1 transgene, and 3) the increased expression of slow/type I isomyosin (beta-MHC, SMLC1, SMLC2) in response to mechanical overload is regulated, in part, transcriptionally.

Analysis of muscle creatine kinase gene regulatory elements in skeletal and cardiac muscles of transgenic mice

Regulatory regions of the mouse muscle creatine kinase (MCK) gene, previously discovered by analysis in cultured muscle cells, were analyzed in transgenic mice. The 206-bp MCK enhancer at nt-1256 was required for high-level expression of MCK-chloramphenicol acetyltransferase fusion genes in skeletal and cardiac muscle; however, unlike its behavior in cell culture, inclusion of the 1-kb region of DNA between the enhancer and the basal promoter produced a 100-fold increase in skeletal muscle activity. Analysis of enhancer control elements also indicated major differences between their properties in transgenic muscles and in cultured muscle cells. Transgenes in which the enhancer right E box or CArG element were mutated exhibited expression levels that were indistinguishable from the wild-type transgene. Mutation of three conserved E boxes in the MCK 1,256-bp 5' region also had no effect on transgene expression in thigh skeletal muscle expression. All these mutations significantly reduced activity in cultured skeletal myocytes. However, the enhancer AT-rich element at nt - 1195 was critical for expression in transgenic skeletal muscle. Mutation of this site reduced skeletal muscle expression to the same level as transgenes lacking the 206-bp enhancer, although mutation of the AT-rich site did not affect cardiac muscle expression. These results demonstrate clear differences between the activity of MCK regulatory regions in cultured muscles cells and in whole adult transgenic muscle. This suggests that there are alternative mechanism of regulating the MCK gene in skeletal and cardiac muscle under different physiological states.

Fast-muscle-specific DNA-protein interactions occurring in vivo at the human aldolase A M promoter are necessary for correct promoter activity in transgenic mice

The human aldolase A tissue-specific M promoter (pM) has served as a model system for identifying pathways that lead to fast-muscle-specialized expression. The current study has delimited the sequences necessary and sufficient for fast-muscle-specific expression in transgenic mice to a short 209-bp fragment extending from bp -164 to +45 relative to the pM transcription start site. Genomic footprinting methods showed that in this proximal region, the same elements that bind muscle nuclear proteins in vitro are involved in DNA-protein interactions in intact muscle nuclei of transgenic mice. Furthermore, these experiments provided the first evidence that different DNA-binding activities exist between slow and fast muscles in vivo. Fast-muscle-specific interactions occur at an element named M1 and at a muscle-specific DNase I-hypersensitive site that was previously detected by in vitro methods. The formation of the muscle-specific DNase I-hypersensitive site reflects binding of proteins to a close element, named M2, which contains a binding site for nuclear factors of the NF1 family. Mutational analysis performed with transgenic mice confirmed the importance of the M1 element for high-level fast-muscle-specific pM activity and suggested that the M2/NF1 element is differently required for correct pM expression in distinct fast muscles. In addition, two other protein binding sites, the MEF3 motif and the USF site, seem to act as stage-specific activators and/or as participants in the establishment of an active chromatin configuration at pM.

Postnatal development and plasticity of specialized muscle fiber characteristics in the hindlimb

Recent progress in defining molecular components of pathways controlling early stages of myogenesis has been substantial, but regulatory factors that govern the striking functional specialization of adult skeletal muscle fibers in vertebrate organisms have not yet been identified. A more detailed understanding of the temporal and spatial patterns by which specialized fiber characteristics arise may provide clues to the identity of the relevant regulatory factors. In this study, we used immunohistochemical, in situ hybridization, and Northern blot analyses to examine the time course and spatial characteristics of expression of myoglobin protein and mRNA during development of the distal hindlimb in the mouse. In adult animals, myoglobin is expressed selectively in oxidative, mitochondria-rich, fatigue-resistant myofibers, and it provides a convenient marker for this particular subset of specialized fibers. We observed only minimal expression of myoglobin in the hindlimb prior to the second day after birth, but a rapid and large (50-fold) induction of this gene in the ensuing neonatal period. Myoglobin expression was limited, however, to fibers located centrally within the limb which coexpress myosin isoforms characteristic of type I, IIA, and IIX fibers. This induction of myoglobin expression within the early postnatal period was accompanied by increased expression of nuclear genes encoding mitochondrial proteins, and exhibited a time course similar to the upregulation of myoglobin and mitochondrial proteins, and exhibited a time course similar to the upregulation of myoglobin and mitochondrial protein expression that can be induced in adult muscle fibers by continuous motor nerve stimulation. This comparison suggests that progressive locomotor activity of neonatal animals may provide signals which trigger the development of the specialized features of oxidative, fatigue-resistant skeletal muscle fibers.

Adaptation of nicotinic acetylcholine receptor, myogenin, and MRF4 gene expression to long-term muscle denervation

Muscle activity alters the expression of functionally distinct nicotinic acetylcholine receptors (nAChR) via regulation of subunit gene expression. Denervation increases the expression of all subunit genes and promotes the expression of embryonic-type (alpha 2 beta delta gamma) nAChRs, while electrical stimulation of denervated muscle prevents this induction. We have discovered that the denervation-induced increases in alpha, beta, gamma, and delta subunit gene expression do not persist in muscles that have been denervated for periods extending beyond a couple of months. However, expression of RNA encoding the epsilon-subunit remains elevated suggesting a return to expression of predominantly adult-type (alpha 2 beta delta epsilon) nAChR in long-term denervated muscles; a finding confirmed by single channel patch-clamp analysis. Since the nAChR subunit genes are regulated by the MyoD family of muscle regulatory factors, and the genes encoding these factors are also induced following short-term muscle denervation, we determined their level of expression in long-term denervated muscle. Although MyoD and myf-5 RNA levels remained elevated, myogenin and MRF4 RNAs were induced only transiently by muscle denervation. Surprisingly, Id-1, a negative regulator of transcription, was gradually induced in denervated muscle with RNA levels peaking about two months after denervation. It is likely that this maintained level of increased Id expression, in conjunction with the returning levels of myogenin and MRF4 expression, account for the reduced level of embryonic receptors in long-term denervated muscle. These changing patterns of gene expression may have important consequences for the ability of muscle to recover function after denervation.

Fast and slow rat muscles degenerate and regenerate differently after whole crush injury

The whole-crush injured rat skeletal muscle was used as a model to explore the regenerating potentialities of fast and slow muscles. Laminin was chosen to follow changes in basal lamina and desmin to visualize new muscular elements; they were revealed by immunofluorescence on cryostat sections of either fast (extensor digitorum longus) or slow (soleus) regenerating muscle. Soleus myolysis was rapid, extensive and heterogeneous. Basal laminae were nearly destroyed. In contrast, extensor digitorum longus maintained its basal lamina framework during myolysis. Soleus reconstruction began early, following the pattern of remaining basal laminae as closely as possible, but regeneration stagnated from day 16 and the regenerated muscle was fibrotic. In extensor digitorum longus, reconstruction progressed slower than in soleus, but regularly from the periphery toward the centre of the muscle. The regenerated extensor digitorum longus showed a quasi-normal structure from day 16. At the end of the process, the elimination of old basal lamina was completed in extensor digitorum longus, but was not achieved in soleus. We propose that the old basal lamina should help the initiation of reconstruction. This new model also underlines the importance of the turnover of basal laminae in muscular regeneration, and will be useful to understand the background of the different regenerative response of both muscles.

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

Contractile proteins are encoded by multigene families, most of whose members are differentially expressed in fast- versus slow-twitch myofibers. This fiber-type-specific gene regulation occurs by unknown mechanisms and does not occur within cultured myocytes. We have developed a transient, whole-animal assay using somatic gene transfer to study this phenomenon and have identified a fiber-type-specific regulatory element within the promoter region of a slow myofiber-specific gene. A plasmid-borne luciferase reporter gene fused to various muscle-specific contractile gene promoters was differentially expressed when injected into slow- versus fast-twitch rat muscle: the luciferase gene was preferentially expressed in slow muscle when fused to a slow troponin I promoter, and conversely, was preferentially expressed in fast muscle when fused to a fast troponin C promoter. In contrast, the luciferase gene was equally well expressed by both muscle types when fused to a nonfiber-type-specific skeletal actin promoter. Deletion analysis of the troponin I promoter region revealed that a 157-bp enhancer conferred slow-muscle-preferential activity upon a minimal thymidine kinase promoter. Transgenic analysis confirmed the role of this enhancer in restricting gene expression to slow-twitch myofibers. Hence, somatic gene transfer may be used to rapidly define elements that direct myofiber-type-specific gene expression prior to the generation of transgenic mice.

MRF4, Myf-5, and myogenin mRNAs in the adaptive responses of mature rat muscle

We studied the possible role of specific muscle regulatory factors (MRF) in the adaptive response to changes in contractile activity in mature skeletal muscle. The tibialis anterior muscle of anesthetized female rats was subjected to low-frequency stimulation, static stretch, or a combination of both. Message levels of MRF were observed after 2 h of activity, and the subsequent 20-h recovery period by slot blot and in situ hybridizations for MRF4, Myf-5, and myogenin. A combination of stimulation and stretch for 2 h increased MRF4 (11.6 +/- 5.3-fold) and Myf-5 (6.6 +/- 1.4-fold). In situ hybridization showed abundance in some regions of the muscle with positive staining near peripheral nuclei of both large and small fibers. Message levels remained high for 30 min and declined to near control levels by 20 h of recovery. Myogenin mRNA levels were unaffected by any manipulations. Neither stretch alone nor 10 Hz of electrical stimulation alone induced a significant increase in MRF. We conclude that myonuclei, and possibly activated myoblasts, increase expression of Myf-5 and MRF4 after a combination of both stimulation and stretch for 2 h.

New fiber formation in rat soleus muscle following administration of denervated muscle extract

A study was made of Wistar rat soleus muscle following intraperitoneal administration of denervated muscle extract over 1 and 2 days. Light microscopy revealed the appearance on fiber surfaces of basophilic satellite structures whose histochemical behaviour differed from that of the parent fiber. Small fibers showing regenerative characteristics were also detected, mainly in the extrafascicular spaces. At ultrastructural examination, activated satellite cells were visible, and there was evidence of splitting in subsarcolemmal regions of apparently hypertrophic muscle fibers. Interstitial cells were occasionally observed, containing structures like myofilaments. The hypothesis is advanced that denervated muscle extract contains substances able to stimulate new fiber formation in adult skeletal muscle.

AML1 is expressed in skeletal muscle and is regulated by innervation

Although most skeletal muscle genes are expressed at similar levels in electrically active, innervated muscle and in electrically inactive, denervated muscle, a small number of genes, including those encoding the acetylcholine receptor, N-CAM, and myogenin, are expressed at significantly higher levels in denervated than in innervated muscle. The mechanisms that mediate electrical activity-dependent gene regulation are not understood, but these mechanisms are likely to be responsible, at least in part, for the changes in muscle structure and function that accompany a decrease in myofiber electrical activity. To understand how muscle activity regulates muscle structure and function, we used a subtractive-hybridization and cloning strategy to identify and isolate genes that are expressed preferentially in innervated or denervated muscle. One of the genes which we found to be regulated by electrical activity is the recently discovered acute myeloid leukemia 1 (AML1) gene. Disruption and translocation of the human AML1 gene are responsible for a form of acute myeloid leukemia. AML1 is a DNA-binding protein, but its normal function is not known and its expression and regulation in skeletal muscle were not previously appreciated. Because of its potential role as a transcriptional mediator of electrical activity, we characterized expression of the AML1 gene in innervated, denervated, and developing skeletal muscle. We show that AML1 is expressed at low levels in innervated skeletal muscle and at 50- to 100-fold-higher levels in denervated muscle. Four AML1 transcripts are expressed in denervated muscle, and the abundance of each transcript increases after denervation. We transfected C2 muscle cells with an expression vector encoding AML1, tagged with an epitope from hemagglutinin, and we show that AML1 is a nuclear protein in muscle. AML1 dimerizes with core-binding factor beta (CBF beta), and we show that CGF beta is expressed at high levels in both innervated and denervated skeletal muscle. PEBP2 alpha, which is structurally related to AML1 and which also dimerizes with CBF beta, is expressed at low levels in skeletal muscle and is up-regulated only weakly by denervation. These results are consistent with the idea that AML1 may have a role in regulating gene expression in skeletal muscle.

AML1 is expressed in skeletal muscle and is regulated by innervation

Although most skeletal muscle genes are expressed at similar levels in electrically active, innervated muscle and in electrically inactive, denervated muscle, a small number of genes, including those encoding the acetylcholine receptor, N-CAM, and myogenin, are expressed at significantly higher levels in denervated than in innervated muscle. The mechanisms that mediate electrical activity-dependent gene regulation are not understood, but these mechanisms are likely to be responsible, at least in part, for the changes in muscle structure and function that accompany a decrease in myofiber electrical activity. To understand how muscle activity regulates muscle structure and function, we used a subtractive-hybridization and cloning strategy to identify and isolate genes that are expressed preferentially in innervated or denervated muscle. One of the genes which we found to be regulated by electrical activity is the recently discovered acute myeloid leukemia 1 (AML1) gene. Disruption and translocation of the human AML1 gene are responsible for a form of acute myeloid leukemia. AML1 is a DNA-binding protein, but its normal function is not known and its expression and regulation in skeletal muscle were not previously appreciated. Because of its potential role as a transcriptional mediator of electrical activity, we characterized expression of the AML1 gene in innervated, denervated, and developing skeletal muscle. We show that AML1 is expressed at low levels in innervated skeletal muscle and at 50- to 100-fold-higher levels in denervated muscle. Four AML1 transcripts are expressed in denervated muscle, and the abundance of each transcript increases after denervation. We transfected C2 muscle cells with an expression vector encoding AML1, tagged with an epitope from hemagglutinin, and we show that AML1 is a nuclear protein in muscle. AML1 dimerizes with core-binding factor beta (CBF beta), and we show that CGF beta is expressed at high levels in both innervated and denervated skeletal muscle. PEBP2 alpha, which is structurally related to AML1 and which also dimerizes with CBF beta, is expressed at low levels in skeletal muscle and is up-regulated only weakly by denervation. These results are consistent with the idea that AML1 may have a role in regulating gene expression in skeletal muscle.