The temporal and cellular expression of c-fos and c-jun in mechanically stimulated rabbit latissimus dorsi muscle

JNK activation in TA and EDL muscle is load-dependent in rats receiving identical excitation patterns

As the excitation-contraction coupling is inseparable during voluntary exercise, the relative contribution of the mechanical and neural input on hypertrophy-related molecular signalling is still poorly understood. Herein, we use a rat in-vivo strength exercise model with an electrically-induced standardized excitation pattern, previously shown to induce a load-dependent increase in myonuclear number and hypertrophy, to study acute effects of load on molecular signalling. We assessed protein abundance and specific phosphorylation of the four protein kinases FAK, mTOR, p70S6K and JNK after 2, 10 and 28 min of a low- or high-load contraction, in order to assess the effects of load, exercise duration and muscle-type on their response to exercise. Specific phosphorylation of mTOR, p70S6K and JNK was increased after 28 min of exercise under the low- and high-load protocol. Elevated phosphorylation of mTOR and JNK was detectable already after 2 and 10 min of exercise, respectively, but greatest after 28 min of exercise, and JNK phosphorylation was highly load-dependent. The abundance of all four kinases was higher in TA compared to EDL muscle, p70S6K abundance was increased after exercise in a load-independent manner, and FAK and JNK abundance was reduced after 28 min of exercise in both the exercised and control muscles. In conclusion, the current study shows that JNK activation after a single resistance exercise is load-specific, resembling the previously reported degree of myonuclear accrual and muscle hypertrophy with repetition of the exercise stimulus.

Plasma membrane disruption (PMD) formation and repair in mechanosensitive tissues

Mammalian cells employ an array of biological mechanisms to detect and respond to mechanical loading in their environment. One such mechanism is the formation of plasma membrane disruptions (PMD), which foster a molecular flux across cell membranes that promotes tissue adaptation. Repair of PMD through an orchestrated activity of molecular machinery is critical for cell survival, and the rate of PMD repair can affect downstream cellular signaling. PMD have been observed to influence the mechanical behavior of skin, alveolar, and gut epithelial cells, aortic endothelial cells, corneal keratocytes and epithelial cells, cardiac and skeletal muscle myocytes, neurons, and most recently, bone cells including osteoblasts, periodontal ligament cells, and osteocytes. PMD are therefore positioned to affect the physiological behavior of a wide range of vertebrate organ systems including skeletal and cardiac muscle, skin, eyes, the gastrointestinal tract, the vasculature, the respiratory system, and the skeleton. The purpose of this review is to describe the processes of PMD formation and repair across these mechanosensitive tissues, with a particular emphasis on comparing and contrasting repair mechanisms and downstream signaling to better understand the role of PMD in skeletal mechanobiology. The implications of PMD-related mechanisms for disease and potential therapeutic applications are also explored.

Adaptive responses of histone modifications to resistance exercise in human skeletal muscle

Exercise training causes epigenetic changes in skeletal muscle, although it is unclear how resistance exercise (RE) affects histone modifications. The present study was carried out to investigate the effects of acute RE and RE training on gene expression profiles and histone modifications in human skeletal muscle. Healthy male adults were assigned to acute RE (n = 9, age = 20.5±4.3yr, BMI = 28.0±6.8kg/m2) or RE training (n = 21, age = 23.7±2.5yr, BMI = 24.2±2.7kg/m2) groups. Biopsy samples were obtained from the vastus lateralis muscle before and three hours after a single bout of acute RE, or 3-days after 10 weeks of RE training. RNA sequencing analysis revealed that 153 genes with GO terms including muscle development, stress response, metabolism, cell death, and transcription factor were significantly up-regulated (+291% vs. pre-acute RE) upon acute RE. Expressions of these genes were also greater (+9.6% vs. pre-RE training, p<0.05) in RE trained subjects. Significant up-regulation of acetylated histone 3 (H3) (+235%) and H3 mono-methylated at lysine 4 (+290%) and tri-methylated at lysine 27 (+849%), whereas down-regulation of H3.3 variant (-39%) distributions relative to total H3 were observed at transcriptionally activated loci after acute RE compared to pre-acute RE levels. Interestingly, the distribution of acetylated H3 was found to be up-regulated as compared to the level of total H3 after RE training (+40%, p<0.05). These results indicate that a single bout of RE drastically alters both gene expressions and histone modifications in human skeletal muscle. It is also suggested that enhanced histone acetylation is closely related to up-regulation of gene expressions after RE training.

A novel role for cardiac ankyrin repeat protein Ankrd1/CARP as a co-activator of the p53 tumor suppressor protein

The muscle ankyrin repeat protein (MARP) family member Ankrd1/CARP is a part of the titin-mechanosensory signaling complex in the sarcomere and in response to stretch it translocates to the nucleus where it participates in the regulation of cardiac genes as a transcriptional co-repressor. Several studies have focused on its structural role in muscle, but its regulatory role is still poorly understood. To gain more insight into the regulatory function of Ankrd1/CARP we searched for transcription factors that could interact and modulate its activity. Using protein array methodology we identified the tumor suppressor protein p53 as an Ankrd1/CARP interacting partner and confirmed their interaction both in vivo and in vitro. We demonstrate a novel role for Ankrd1/CARP as a transcriptional co-activator, moderately up regulating p53 activity. Furthermore, we show that p53 operates as an upstream effector of Ankrd1/CARP, by up regulating the proximal ANKRD1 promoter. Our findings suggest that, besides acting as a transcriptional co-repressor, Ankrd1/CARP could have a stimulatory effect on gene expression in cultured skeletal muscle cells. It is probable that Ankrd1/CARP has a role in the propagation of signals initiated by myogenic regulatory factors (MRFs) during myogenesis.

Effects of fatiguing jumping exercise on mRNA expression of titin-complex proteins and calpains

Eccentric exercise induced by electrostimulation increases mRNA expression of titin-complex proteins in rodent skeletal muscle. In this study, mRNA expression of titin, muscle LIM protein (MLP), cardiac ankyrin repeat protein (CARP), ankyrin repeat domain protein 2 (Ankrd2), diabetes-related ankyrin repeat protein (DARP), and calcium-activated proteinases, calpains, were investigated in human skeletal muscle after fatiguing jumping exercise. Fatiguing jumping exercise did not change mRNA expression of titin, DARP, calpain 1, or calpain 3. MLP, Ankrd2 and calpain 2 mRNA levels were increased 2 days postexercise. CARP mRNA level was already elevated 30 min and remained elevated 2 days postexercise. Increased mRNA expression of MLP, CARP, and Ankrd2, observed for the first time in human skeletal muscle, may be part of the signaling activated by physical exercise. The rapid increase in the level of CARP mRNA nominates CARP as one of the first genes to respond to exercise. The increase in the mRNA level of calpain 2 suggests its involvement in myofiber remodeling after strenuous jumping exercise.

Influence of mechanical and biological signals on gene expression in human MG-63 cells: evidence for a complex interplay between hydrostatic compression and vitamin D3 or TGF-beta1 on MMP-1 and MMP-3 mRNA levels

Biological mediators can influence the activity and differentiation of bone cells. 1,25-dihydroxy-vitamin D3 (1,25-(OH)2D3) is known to induce differentiation of precursors into mature osteoblasts, and transforming growth factor-beta1 (TGF-beta1) can modulate the activity of bone cells leading to alterations in proliferation and gene expression patterns. Bone-derived cells were loaded via intermittent cyclic hydrostatic pressure (icHP) on cells under basal conditions and in the presence of 1,25-(OH)2D3 or TGF-beta1. Evaluating the effects of loading on the cells allowed for a comparison to be made between responsiveness to biomechanical and biochemical stimuli and their potential interplay. The effects of icHP on mRNA levels for the specific genes involved in bone remodelling and differentiation were measured in MG-63 cells using reverse transcription-polymerase chain reaction (RT-PCR). The mRNA levels for matrix metalloproteinase-1 and -3 (MMP-1 and MMP-3) were significantly, and uniquely, increased (p < 0.001) in cells exposed to icHP under serum-free conditions for 4-12 h. However, mRNA levels for MMP-3, but not MMP-1, were significantly enhanced in cells subjected to static hydrostatic pressure (HP). Treatment of cells with 1,25-(OH)2D3 resulted in increased (p < 0.001) mRNA levels for osteocalcin and decreased (p < 0.001) mRNA levels for both MMP-1 and MMP-3. In cells exposed to icHP and 1,25-(OH)2D3, the mRNA levels for both MMP-1 and MMP-3 were elevated (p < 0.001) compared with hormone alone, but not to the same degree (p < 0.01) as cells subjected to icHP alone. Addition of TGF-beta1 to cells led to increases in cell proliferation and expression of collagen I, as well as decreases in expression of osteocalcin and MMP-1 and MMP-3. Exposure of cells to icHP and TGF-beta1 again led to unique and significant increases in expression of MMP-1 and MMP-3. No changes in mRNA levels for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) or any of the other 9 genes assessed, including those for MMP-2 and MMP-13, were detected under any of the conditions described. Therefore, icHP can induce alterations in mRNA levels for a specific subset of genes in both premature and mature osteoblasts. Such stimuli can modulate the impact of potent biological mediators in defining patterns of gene expression by bone cells and potentially modify function in vivo.

Gene expression in skeletal tissues: application of laser capture microdissection

Tissue differentiation is based on the expression of transcription factors, receptors for cytokines, and nuclear receptors that regulate a specific phenotype. The purpose of this study was to select cells from various skeletal tissues in order to analyse differential gene expression of cells in the native environment in vivo. It is a difficult task to obtain cells from skeletal tissues, such as cartilage, periost, bone and muscle, that are structured together and do not exist as individual organs. We used laser capture microdissection which permits the selection and isolation of individual cells from tissue sections. The RNA isolated from these tissues was used for reverse transcriptase-polymerase chain reactions for molecular analysis. We analysed the expression of transcription factors (cFOS, cbfa1, MyoD), receptors for cytokines, nuclear receptors, alkaline phosphatase and the structural proteins osteocalcin and collagen II. The results obtained demonstrate differential patterns of gene expression according to the tissue arrangement in their native in vivo environment, with reliable interpretation of the functions of the analysed genes in the context of intact skeletal tissue physiology.

Ageing and activity: their effects on the functional reserve capacities of the heart and vascular smooth and skeletal muscles

During perinatal life striated muscles grow through the acquisition of more contractile cells (myocytes or fibres) followed by their postnatal enlargement (i.e. hypertrophy). In the ageing adult these events are reversed, with a progressive loss of myocytes that cannot be fully compensated despite the presence of cell renewal systems or reactive myocyte hypertrophy. Hence the functional reserve capacities of the heart and skeletal muscles decline with age. This is probably a consequence of physiological ageing and diminished levels of physical activity. As a result daily tasks once taken for granted become progressively more difficult, and eventually impossible, to perform. For example, sufficient coordinated absolute muscle force is required for an individual to rise from a chair or climb stairs, and the reserve capacity of the heart is a major determinant of an individual's ability to remain active and cope with daily stresses and illnesses. Long-term participation in endurance-based activities helps to preserve cardiac reserve, and has both direct and indirect beneficial effects on vascular smooth muscle and health preservation within the cardiovascular system. In contrast, this type of activity does little to protect skeletal muscles against the age-related losses of fast-twitch fibres, small motor units, overall muscle mass and power output. While resistance exercise promotes fibre hypertrophy in skeletal muscles, and to a lesser extent in myocytes of the heart, the explosive power of muscles still declines with age. Hence, while physical activity is important in attenuating age-related changes in muscle function and its reserve capacity, it delays rather than prevents the deleterious effects of ageing per se. Despite this, in a culture where inactivity has become an accepted part of life we still need to explore in greater detail the benefits of habitual physical activity, and use this information as a community-based educational tool to help prevent or delay cardiovascular disease, obesity, arthritis and the frailty associated with old age.

Rapid muscle-specific gene expression changes after a single bout of eccentric contractions in the mouse

Eccentric contractions (ECs), in which a muscle is forced to lengthen while activated, result in muscle injury and, eventually, muscle strengthening and prevention of further injury. Although the mechanical basis of EC-induced injury has been studied in detail, the biological response of muscle is less well characterized. This study presents the development of a minimally invasive model of EC injury in the mouse, follows the time course of torque recovery after an injurious bout of ECs, and uses Affymetrix microarrays to compare the gene expression profile 48 h after ECs to both isometrically stimulated muscles and contralateral muscles. Torque dropped by ∼55% immediately after the exercise bout and recovered to initial levels 7 days later. Thirty-six known genes were upregulated after ECs compared with contralateral and isometrically stimulated muscles, including five muscle-specific genes: muscle LIM protein (MLP), muscle ankyrin repeat proteins (MARP1 and -2; also known as cardiac ankyrin repeat protein and Arpp/Ankrd2, respectively), Xin, and myosin binding protein H. The time courses of MLP and MARP expression after the injury bout (determined by quantitative real-time polymerase chain reaction) indicate that these genes are rapidly induced, reaching a peak expression level of 6–11 times contralateral values 12–24 h after the EC bout and returning to baseline within 72 h. Very little gene induction was seen after either isometric activation or passive stretch, indicating that the MLP and MARP genes may play an important and specific role in the biological response of muscle to EC-induced injury.

Molecular responses of human muscle to eccentric exercise

We examined the effect of eccentric exercise on the transcriptome of skeletal muscle in three male human volunteers who performed 300 concentric contractions with one leg and 300 eccentric contractions with the opposite leg. Vastus lateralis muscle biopsies were taken from both legs at 4-8 h after exercise, and expression was profiled by using 12000 gene Affymetrix U95Av2 microarrays. We found a high concordance of expression responses to eccentric contractions between our human and rat data from a previous study (Chen YW, Nader GA, Baar KR, Fedele MJ, Hoffman EP, and Esser KA. J Physiol 545: 27-41, 2002) ( approximately 50% of gene expression changes shared between species). Potential human-specific changes included greater inflammatory responses [chemokine (C-C motif) ligand 2, C/EBP delta, and IL-1 receptor] and vascular remodeling (tenascin C and lipocortin II). Induction of c-fos and lipocortin II were confirmed at the protein level, with c-fos localized to myofiber nuclei and lipocortin II to intramuscular capillaries. We also confirmed the eccentric-induced expression of six transcripts by quantitative RT-PCR (cardiac ankyrin-repeated protein, chemokine ligand 2, CCAAT/enhancer binding protein delta, IL-1 receptor, tenascin C, and cysteine-rich angiogenic inducer 61). These data provide the first characterization of the transcriptional response of skeletal muscle to eccentric exercise in humans and represent a preliminary step in understanding the molecular processes underlying muscle remodeling (including a new focus on rapid changes in the capillary bed) and inflammatory responses after damaging lengthening contractions.

Depolarization-induced slow calcium transients activate early genes in skeletal muscle cells

The signaling mechanisms by which skeletal muscle electrical activity leads to changes in gene expression remain largely undefined. We have reported that myotube depolarization induces calcium signals in the cytosol and nucleus via inositol 1,4,5-trisphosphate (IP(3)) and phosphorylation of both ERK1/2 and cAMP-response element-binding protein (CREB). We now describe the calcium dependence of P-CREB and P-ERK induction and of the increases in mRNA of the early genes c-fos, c-jun, and egr-1. Increased phosphorylation and early gene activation were maintained in the absence of extracellular calcium, while the increase in intracellular calcium induced by caffeine could mimic the depolarization stimulus. Depolarization performed either in the presence of the IP(3) inhibitors 2-aminoethoxydiphenyl borate or xestospongin C or on cells loaded with BAPTA-AM, in which slow calcium signals were abolished, resulted in decreased activation of the early genes examined. Both early gene activation and CREB phosphorylation were inhibited by ERK phosphorylation blockade. These data suggest a role for calcium in the transcription-related events that follow membrane depolarization in muscle cells.

Stretch-activated ion channels and c-fos expression remain active after repeated eccentric bouts

This study was undertaken to measure the response of stretch-activated ion channels (SAC) and transcript levels of the oncogene c-fos to separate bouts of eccentric contractions (EC). It was hypothesized that SAC in rat skeletal muscle would contribute to resting membrane potential depolarization after separate repeated bouts of EC. Blockage of SAC during an EC training regime also tested the necessity of SAC for a training response. It was also hypothesized that transcript levels of c-fos would be maximally elevated after the first exposure to EC and diminish with repeated exposures. The results indicate less depolarization after multiple bouts of EC, which could be reversed by blocking the SAC. Transcript levels of c-fos were elevated to a similar degree after either a single or multiple exposures to EC. EC training resulted in significant increases in contractile force and muscle wet and dry weights in nontreated animals. Training in the presence of the SAC-blocker streptomycin produced similar changes in contractile force without changes in muscle weight. SAC and c-fos are activated after several exposures to EC and therefore remain as possible signals in EC training responses.

Effect of protein kinase inhibitors on the stretch-elicited c-Fos and c-Jun up-regulation in human PDL osteoblast-like cells

Osteoblastic cells transduce signals of mechanical loading that plays a key role in maintaining bone formation. In an attempt to elucidate the biochemical events associated with the conversion of mechanical stress to biological outcome, we examined cultured human periodontal ligament (hPDL) osteoblastic cells exposed to continuous stretch, in terms of cellular parameters correlating known signaling cascades to the initial phase of osteoblast-specific transcriptional control. Time-course experiments revealed that mechanical stretch-loaded hPDL cells exhibit a very rapid and relatively sustained increase in the abundance of the immediate-early gene products, c-Fos and c-Jun, components of the activator protein-1 (AP-1) transcription factor. Moreover, this increase in protein levels was accompanied by hyperphosphorylation and thereby potentiation of c-Jun, the principal modulator of AP-1 activity. Importantly, these inductive effects were partly or completely abolished by pre-incubating the cells with SB 203580, PD 098059, and the novel compound Y-27632, inhibitors of p38 mitogen-activated protein kinase (MAPK), MAPK kinase (MEK), and Rho-associated protein kinase (RhoK), respectively. These results consolidate AP-1 as the pivotal downstream effector in the early response of hPDL cells to continuous mechanical stretching, via the coordinate stimulation of de novo synthesis and post-translational regulation of AP-1 proteins. This "integrating" function of AP-1 is mediated through a mechanotransduction circuit that incorporates elements of well-defined upstream signaling protein kinase systems.

Dynamics of stimulation-induced muscle adaptation: insights from varying the duty cycle

We sought to gain insight into the dynamics of the signalling process that initiates adaptive change in mammalian skeletal muscles in response to chronic neuromuscular stimulation. Programmable miniature stimulators were implanted into rabbits and used to impose one of the following patterns on the dorsiflexors of one ankle: 10 Hz delivered in equal on/off periods of 30 s, 30 min, or 12 h (all equivalent in terms of aggregate impulse activity to continuous 5 Hz). Two further groups received continuous stimulation at 5 Hz or 10 Hz. In every case the stimulation pattern was maintained continuously for 6 weeks. Tibialis anterior muscles stimulated intermittently with equal on/off periods of 30 s, 30 min and 12 h had contractile characteristics that were significantly slower than the contralateral, unstimulated muscles but did not differ from those of muscles stimulated continuously at 5 Hz. Muscles stimulated continuously at 10 Hz were significantly slower than either contralateral muscles or muscles stimulated with any of the other patterns. Corresponding changes were seen in myosin heavy chain isoform composition. The fatigue index, defined as the fraction of tension remaining after 5 min of a standard fatigue test, was 0.4 for muscles in the contralateral group but equal to or greater than 0.85 for muscles of all the stimulated groups. These results were interpreted with the help of a simple model of the growth and decay of a putative signalling substance based on first order kinetics. The model suggests a rate constant for the accumulation of the signalling substance that is greater than 30 h(-1), and a rate constant for its removal that is greater than 50 h(-1).

beta-adrenergic receptor population is up-regulated by increased cyclic adenosine monophosphate concentration in chicken skeletal muscle cells in culture

Skeletal muscle hypertrophy is promoted in vivo by administration of beta-adrenergic receptor (betaAR) agonists. Chicken skeletal muscle cells were treated with 1 microM isoproterenol, a strong betaAR agonist, between days 7 and 10 in culture. betaAR population increased by approximately 40% during this treatment; however, the ability of the cells to synthesize cyclic adenosine monophosphate (cAMP) was diminished by twofold. Neither the basal concentration of cAMP nor the quantity of myosin heavy chain (MHC) was affected by the 3-d exposure to isoproterenol. To understand further the relationship between intracellular cAMP levels, betaAR population, and muscle protein accumulation, intracellular cAMP levels were artificially elevated by treatment with 0-10 betaM forskolin for 3 d. The basal concentration of cAMP in forskolin-treated cells increased up to sevenfold in a dose-dependent manner. Increasing concentrations of forskolin also led to an increase in betaAR population, with a maximum increase of approximately 40-60% at 10 microM forskolin. A maximum increase of 40-50% in the quantity of MHC was observed at 0.2 microM forskolin, but higher concentrations of forskolin reduced the quantity of MHC back to control levels. At 0.2 microM forskolin, intracellular levels of cAMP were higher by approximately 35%, and the betaAR population was higher by approximately 30%. Neither the number of muscle nuclei fused into myotubes nor the percentage of nuclei in myotubes was affected by forskolin at any of the concentrations studied.

Compressive compared with tensile loading of medial collateral ligament scar in vitro uniquely influences mRNA levels for aggrecan, collagen type II, and collagenase

To test the hypothesis that loading conditions can be used to engineer early ligament scar behaviors, we used an in vitro system to examine the effect that cyclic hydrostatic compression and cyclic tension applied to 6-week rabbit medial collateral ligament scars had on mRNA levels for matrix molecules, collagenase, and the proto-oncogenes c-fos and c-jun. Our specific hypothesis was that tensile stress would promote more normal mRNA expression in ligament whereas compression would lead to higher levels of mRNA for cartilage-like molecules. Femur (injured medial collateral ligament)-tibia complexes were subjected to a hydrostatic pressure of 1 MPa or a tensile stress of 1 MPa of 0.5 Hz for 1 minute followed by 14 minutes of rest. On the basis of a preliminary optimization experiment, this 15-minute testing cycle was repeated for 4 hours. Semiquantitative reverse transcription-polymerase chain reaction analysis was performed for mechanically treated medial collateral ligament scars with use of rabbit specific primer sets for types I, II, and III collagen, decorin, biglycan, fibromodulin, versican, aggrecan, collagenase, c-fos, c-jun, and a housekeeping gene, glyceraldehyde-3-phosphate dehydrogenase. Cyclic hydrostatic compression resulted in a statistically significant increase in mRNA levels of type-II collagen (171% of nonloaded values) and aggrecan (313% of nonloaded values) but statistically significant decreases in collagenase mRNA levels (35% of nonloaded values). Cyclic tension also resulted in a statistically significant decrease in collagenase mRNA levels (66% of nonloaded values) and an increase in aggrecan mRNA levels (458% of nonloaded values) but no significant change in the mRNA levels for the other molecules. The results show that it is possible to alter mRNA levels for a subset of genes in scar tissue by supplying unique mechanical stimuli in vitro and thus that further investigation of scar engineering for potential reimplantation appears feasible.

Identification of Ankrd2, a novel skeletal muscle gene coding for a stretch-responsive ankyrin-repeat protein

Mechanically induced hypertrophy of skeletal muscles involves shifts in gene expression leading to increases in the synthesis of specific proteins. Full characterization of the regulation of muscle hypertrophy is a prerequisite for the development of novel therapies aimed at treating muscle wasting (atrophy) in human aging and disease. Using suppression subtractive hybridization, cDNAs corresponding to mRNAs that increase in relative abundance in response to mechanical stretch of mouse skeletal muscles in vivo were identified. A novel 1100-bp transcript was detected exclusively in skeletal muscle. This exhibited a fourfold increase in expression after 7 days of stretch. The transcript had an open reading frame of 328 amino acids encoding an ATP/GTP binding domain, a nuclear localization signal, two PEST protein-destabilization motifs, and a 132-amino-acid ankyrin-repeat region. We have named this gene ankyrin-repeat domain 2 (stretch-responsive muscle) (Ankrd2). We hypothesize that Ankrd2 plays an important role in skeletal muscle hypertrophy.

Reversible Ca2+-induced fast-to-slow transition in primary skeletal muscle culture cells at the mRNA level

1. The adult fast character and a Ca2+-inducible reversible transition from a fast to a slow type of rabbit myotube in a primary culture were demonstrated at the mRNA level by Northern blot analysis with probes specific for different myosin heavy chain (MyHC) isoforms and enzymes of energy metabolism. 2. No non-adult MyHC isoform mRNA was detected after 22 days of culture. After 4 weeks of culture the fast MyHCIId mRNA was strongly expressed while MyHCI mRNA was virtually absent, indicating the fast adult character of the myotubes in the primary skeletal muscle culture. 3. The data show that a fast-to-slow transition occurred in the myotubes at the level of MyHC isoform gene expression after treatment with the Ca2+ ionophore A23187. The effects of ionophore treatment were decreased levels of fast MyHCII mRNA and an augmented expression of the slow MyHCI gene. Changes in gene expression started very rapidly 1 day after the onset of ionophore treatment. 4. Levels of citrate synthase mRNA increased and levels of glyceraldehyde 3-phosphate dehydrogenase mRNA decreased during ionophore treatment. This points to a shift from anaerobic to oxidative energy metabolism in the primary skeletal muscle culture cells at the level of gene expression. 5. Withdrawal of the Ca2+ ionophore led to a return to increased levels of MyHCII mRNA and decreased levels of MyHCI mRNA, indicating a slow-to-fast transition in the myotubes and the reversibility of the effect of ionophore on MyHC isoform gene expression.

Selective activation in the MAPK pathway by Hg(II) in precision-cut rabbit renal cortical slices

The kidneys are the primary organ for the accumulation and toxicity of inorganic mercury. In these studies the molecular response of precision-cut rabbit renal cortical slices to low levels of inorganic mercury was examined. Cortical slices (275 microm) were obtained from 1.0 kg NZW rabbits and exposed to mercuric chloride [Hg(II)] at concentrations of 0.01-10 microM for 2-8 h. Overt cytotoxicity, as assessed by intracellular K(+) levels, was not observed following exposure to these concentrations of Hg(II). However, an induction of heme-oxygenase-1 (Hsp32) was seen following a 2-h challenge to Hg(II). A dose-dependent induction of the DNA binding activity of the AP-1 transcription factor after 4 h of Hg(II) exposure correlated with a dose-dependent enhancement of c-jun gene expression following 2 h of Hg(II) exposure. Additionally, an increase in phosphorylated c-Jun NH(2)-terminal protein kinase (JNK) was observed following 2 h of Hg(II) exposure. These results suggest activation of the mitogen-activated protein (MAP) signal transduction pathway, specifically the c-Jun NH(2)-terminal protein kinase (JNK) pathway. No changes were observed, however, in the DNA binding activity of ATF2 and Elk-1, transcription factors involved in both the JNK and p38 pathways of MAP signal transduction, nor in the gene expression of c-myc. This selectivity of alterations in molecular signaling suggests an acute response in signal transduction, specifically activation of the JNK pathway in renal tissue following exposure to nanomolar concentrations of Hg(II).

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.

Prolonged passive stretch of rat soleus muscle provokes an increase in the mRNA levels of the muscle regulatory factors distributed along the entire length of the fibers

The mRNA levels of the adult and the neonatal sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPases (SERCA1a and SERCA1b, respectively) and those of the muscle regulatory factors (MRFs: myoD, myf-5, myogenin, MRF4) have been assessed by RT PCR in rat soleus muscles immobilized for 3 days in an extended position (passive stretch). The transcript level of the fast type SERCA1a Ca(2+)-transport ATPase decreased to half of its normal value, whereas that of neonatal SERCA1b isoform increased 5-fold above control in stretched muscles. Immunostaining of muscle cross sections showed that the fraction of fibers expressing the SERCA1a protein was decreased evenly along the length of the stretched muscles indicating that a transformation occurred of fast fibers to slow ones. The mRNA levels of MRFs were elevated 3- to 6-fold above the normal level and were distributed evenly along the length of the stretched muscles. However in the controls these transcripts were more abundant at both ends of the muscle. The stretch increased the level of myoD and immunocytochemistry showed the expression of myoD protein in a number of nuclei of the stretched muscles whereas it was practically undetectable by this method in the control muscles. Western blotting did not indicate a significant stretch-induced increase in the level of the myogenin protein, in spite of the fact that immunocytochemistry tended to show more myogenin-positive nuclei in stretched muscles as compared to the controls. These data indicate that after 3 days of passive stretch the central and the terminal parts of the soleus muscle adapt similarly by increasing the levels of the MRFs, by decreasing the overall levels of the fast SERCA1-type of ATPase and by partially re-establishing a neonatal mode of alternative SERCA1 transcript splicing resulting in an increased SERCA1b/1a ratio.

Temporary disruption of the plasma membrane is required for c-fos expression in response to mechanical stress

Mechanically stressed cells display increased levels of fos message and protein. Although the intracellular signaling pathways responsible for FOS induction have been extensively characterized, we still do not understand the nature of the primary cell mechanotransduction event responsible for converting an externally acting mechanical stressor into an intracellular signal cascade. We now report that plasma membrane disruption (PMD) is quantitatively correlated on a cell-by-cell basis with fos protein levels expressed in mechanically injured monolayers. When the population of PMD-affected cells in injured monolayers was selectively prevented from responding to the injury, the fos response was completely ablated, demonstrating that PMD is a requisite event. This PMD-dependent expression of fos protein did not require cell exposure to cues inherent in release from cell-cell contact inhibition or presented by denuded substratum, because it also occurred in subconfluent monolayers. Fos expression also could not be explained by factors released through PMD, because cell injury conditioned medium failed to elicit fos expression. Translocation of the transcription factor NF-kappaB into the nucleus may also be regulated by PMD, based on a quantitative correlation similar to that found with fos. We propose that PMD, by allowing a flux of normally impermeant molecules across the plasma membrane, mediates a previously unrecognized form of cell mechanotransduction. PMD may thereby lead to cell growth or hypertrophy responses such as those that are present normally in mechanically stressed skeletal muscle and pathologically in the cardiovascular system.

Tensile force in limb lengthening: histogenesis or only mechanical elongation

Although bone response can be evaluated by radiography, there have been no reports in human confirming formation of new soft tissue in limb lengthening. This study evaluated the tensile force between pin clamps in 14 lower limb lengthenings. Legs were lengthened 0.5 mm every 12 hours and the tensile was measured continuously. The tensile force increased simultaneously with each lengthening and decreased gradually. However, the reduction rate of tensile force during the nighttime (120+/-22%) was significantly higher than that during the daytime (72+/-10%). This differed from the stress relaxation phenomenon shown by viscoelastic material and suggested the presence of other phenomena such as histogenesis.

A regulatory element of the human keratin 18 gene with AP-1-dependent promoter activity

The human keratin 18 (K18) gene is expressed in a restricted but diverse subset of differentiated epithelial tissues and carcinomas. The 10-kilobase pair K18 gene contains all of the genetic information necessary for tissue-specific, copy number-dependent and integration site-independent expression in transgenic mice. We identified a 100-base pair regulatory element that activates the K18 proximal promoter in the presence of the previously identified first intron enhancer. Deletion of the element greatly diminished K18 expression. This regulatory element also has cryptic, AP-1-dependent promoter activity in the absence of the normal promoter, which results in 10-40-fold higher levels of K18 RNA expression in transgenic mice. The high activity of this cryptic promoter is dependent upon the first intron enhancer. These experiments define interactive regulatory regions of the K18 gene that modulate expression in diverse epithelial cell types and identify an unusual regulatory element with promoter activity that may be useful for high level heterologous gene expression in transgenic animals.

Mechanical loading regulates expression of talin and its mRNA, which are concentrated at myotendinous junctions

The hypothesis that mechanical loading regulates talin expression in developing and adult muscle was tested using in vitro and in vivo models. Talin was selected for study because it is a key structural link between the cytoskeleton and cell membrane. In the in vitro model, C2C12 myotubes were subjected to cyclic strains for 48 h. In the in vivo model, rat hindlimb muscles were unloaded for 10 days, then reloaded for 2 days. Cyclic loading of myotubes resulted in significant increases in the quantity of talin (68%) and its 190-kDa proteolytic fragment (70%), as well as talin mRNA (180%), relative to unloaded myotube cultures. Similarly, talin concentration and its mRNA increased by 68 and 136%, respectively, in soleus muscles reloaded for 2 days relative to ambulatory controls. Immunohistochemistry and in situ RT-PCR showed that talin and its mRNA are concentrated and colocalized at myotendinous junctions. Thus these findings indicate that increased mechanical loading promotes talin synthesis, which occurs principally at myotendinous junctions, according to talin mRNA distribution.

Regulation of cardiovascular collagen deposition by mechanical forces

Collagen is a vital component of the extracellular matrix of both the heart and blood vessel walls. It acts as a scaffold to maintain myocardial shape and permit an even distribution of force, and plays a crucial role in the mechanical properties of the blood vessels. Under normal circumstances, collagen is continually being synthesized and degraded throughout life. Increased mechanical stress, which causes myocardial hypertrophy and vessel wall thickening, stimulates collagen turnover. If collagen is deposited in excess (fibrosis), tissue function can be compromised. An understanding of the mechanisms of 'mechanosignal transduction' involved in this process will enable therapeutic approaches to be devised that will prevent inappropriate collagen deposition and thereby help to preserve function.

Expression of fos and jun genes in human skeletal muscle after exercise

It is believed that the induction of the fos and jun gene family of transcription factors might be at the origin of genetic events leading to the differential regulation of muscle-specific genes. We have investigated the effect of a 30-min running bout in untrained subjects on the expression of the mRNAs of all members of the fos and jun gene families, including c-fos, fosB, fosBdel, fra-1, and fra-2 as well as c-jun, junB, and junD. While the fos family members were transiently upregulated 10- to 20-fold (an exception being fra-2) the induction of the jun family members was up to 3-fold only. The induction of c-fos could also be demonstrated at the protein level. Both c-fos and c-jun mRNAs were coinduced in muscle fiber nuclei. The induction was not restricted to a particular fiber type, as expected from established muscle fiber recruitment schemes, but followed a "patchy" pattern confined to certain regions of the muscle. The signals leading to the expression of these immediate early genes are therefore unclear.

Induction of c-fos gene by mercury chloride in LLC-PK1 cells

The c-fos, a member of the immediate early genes, has been reported to be expressed in the renal proximal tubule in response to ischemic and toxic injury. In the present study, effects of mercury chloride (HgCl2) on the expression of c-fos were examined in LLC-PK1 cells. The reverse transcription polymerase chain reaction (RT-PCR) analysis for the semi-quantification of mRNA showed that the treatment of 20 microM HgCl2, markedly increased c-fos mRNA levels. The level of c-fos mRNA began to increase after a 30-min exposure, peaked at 1 h and then returned to the control level at 8 h. The HgCl2-induced c-fos expression was abolished completely by actinomycin-D, indicating it was due to transcriptional activation of the gene. Western blotting immunodetection revealed accumulation of c-Fos protein after 1 h exposure to 20 microM HgCl2. The cytotoxicity of HgCl2 as assayed by mitochondrial dehydrogenase activity (MTT conversion) was observed after 18 h exposure but not at 0.5-8 h. Also, the decrease in cell viability was accompanied with DNA fragmentation, which is characteristic of apoptosis. The present results showed that HgCl2 could induce the early expression of c-fos gene in a renal epithelial cell line.

Mechanical properties of rabbit latissimus dorsi muscle after stretch and/or electrical stimulation

The work loop technique was used to measure the mechanical performance in situ of the latissimus dorsi (LD) muscles of rabbits maintained under fentanyl anesthesia. After 3 wk of incrementally applied stretch the LD muscles were 36% heavier, but absolute power output (195 mW/muscle) was not significantly changed relative to that of external control muscle (206 mW). In contrast, continuous 10-Hz electrical stimulation reduced power output per kilogram of muscle >75% after 3 or 6 wk and muscle mass by 32% after 6 wk. When combined, stretch and 10-Hz electrical stimulation preserved or increased the mass of the treated muscles but failed to prevent an 80% loss in maximum muscle power. However, this combined treatment increased fatigue resistance to a greater degree than electrical stimulation alone. These stretched/stimulated muscles, therefore, are more suitable for cardiomyoplasty. Nonetheless, further work will be necessary to find an ideal training program for this surgical procedure.

Changes in nuclear protein composition in response to chronic electrical stimulation of skeletal muscle

The adaptive response of skeletal muscle to increased functional demand involves phenotypic changes that affect contractile properties, energy metabolism and calcium kinetics. Some of these changes are known to be initiated at a pre-translational level, but the underlying regulatory mechanisms have not yet been identified. In this study we used chronic electrical stimulation (10 Hz, continuous) to initiate fast-to-slow muscle fibre-type transformation, and two-dimensional electrophoresis (2-DE) to assess changes in nuclear protein composition after 24 and 72 h. We report an early and sustained increase in the level of a 3 kDa protein in stimulated fast muscle (n = 6). The presence of the same protein in control slow muscle is consistent with a possible functional role in the determination of the slow phenotype.

Induction of c-fos expression by tributyltin in PC12 cells: involvement of intracellular Ca(2+)

The effects of tributyltin on the expression of immediate early genes, c-fos and c-jun, were examined in PC12 cells using reverse transcription polymerase chain reaction analysis. Tributyltin at concentrations of more than 0.4 μM induced the expression of c-fos after 15 min exposure. The induction of c-fos was accompanied with c-jun expression. Tributyltin-induced c-fos expression was abolished completely by actinomycin D, indicating it was due to transcriptional activation of the gene. Chelation of intracellular Ca(2+) suppressed the expression of c-fos markedly, while removal of external Ca(2+) did not. These results suggest that Ca(2+) mobilized from intracellular stores played a major role in the tributyltin-induced transcriptional activation of c-fos in PC12 cells.