Localization of myogenin, c-fos, c-jun, and muscle-specific gene mRNAs in regenerating rat skeletal muscle

Prolonged FOS activity disrupts a global myogenic transcriptional program by altering 3D chromatin architecture in primary muscle progenitor cells

BACKGROUND

The AP-1 transcription factor, FBJ osteosarcoma oncogene (FOS), is induced in adult muscle satellite cells (SCs) within hours following muscle damage and is required for effective stem cell activation and muscle repair. However, why FOS is rapidly downregulated before SCs enter cell cycle as progenitor cells (i.e., transiently expressed) remains unclear. Further, whether boosting FOS levels in the proliferating progeny of SCs can enhance their myogenic properties needs further evaluation.

METHODS

We established an inducible, FOS expression system to evaluate the impact of persistent FOS activity in muscle progenitor cells ex vivo. We performed various assays to measure cellular proliferation and differentiation, as well as uncover changes in RNA levels and three-dimensional (3D) chromatin interactions.

RESULTS

Persistent FOS activity in primary muscle progenitor cells severely antagonizes their ability to differentiate and form myotubes within the first 2 weeks in culture. RNA-seq analysis revealed that ectopic FOS activity in muscle progenitor cells suppressed a global pro-myogenic transcriptional program, while activating a stress-induced, mitogen-activated protein kinase (MAPK) transcriptional signature. Additionally, we observed various FOS-dependent, chromosomal re-organization events in A/B compartments, topologically associated domains (TADs), and genomic loops near FOS-regulated genes.

CONCLUSIONS

Our results suggest that elevated FOS activity in recently activated muscle progenitor cells perturbs cellular differentiation by altering the 3D chromosome organization near critical pro-myogenic genes. This work highlights the crucial importance of tightly controlling FOS expression in the muscle lineage and suggests that in states of chronic stress or disease, persistent FOS activity in muscle precursor cells may disrupt the muscle-forming process.

A transcriptionally repressed quiescence program is associated with paused RNAPII and is poised for cell cycle reentry

Adult stem cells persist in mammalian tissues by entering a state of reversible quiescence/ G0, associated with low transcription. Using cultured myoblasts and muscle stem cells, we report that in G0, global RNA content and synthesis are substantially repressed, correlating with decreased RNA Polymerase II (RNAPII) expression and activation. Integrating RNAPII occupancy and transcriptome profiling, we identify repressed networks and a role for promoter-proximal RNAPII pausing in G0. Strikingly, RNAPII shows enhanced pausing in G0 on repressed genes encoding regulators of RNA biogenesis (Nucleolin, Rps24, Ctdp1); release of pausing is associated with their increased expression in G1. Knockdown of these transcripts in proliferating cells leads to induction of G0 markers, confirming the importance of their repression in establishment of G0. A targeted screen of RNAPII regulators revealed that knockdown of Aff4 (positive regulator of elongation) unexpectedly enhances expression of G0-stalled genes and hastens S phase; NELF, a regulator of pausing appears to be dispensable. We propose that RNAPII pausing contributes to transcriptional control of a subset of G0-repressed genes to maintain quiescence and impacts the timing of the G0-G1 transition.

Mammalian and Invertebrate Models as Complementary Tools for Gaining Mechanistic Insight on Muscle Responses to Spaceflight

Bioinformatics approaches have proven useful in understanding biological responses to spaceflight. Spaceflight experiments remain resource intensive and rare. One outstanding issue is how to maximize scientific output from a limited number of omics datasets from traditional animal models including nematodes, fruitfly, and rodents. The utility of omics data from invertebrate models in anticipating mammalian responses to spaceflight has not been fully explored. Hence, we performed comparative analyses of transcriptomes of soleus and extensor digitorum longus (EDL) in mice that underwent 37 days of spaceflight. Results indicate shared stress responses and altered circadian rhythm. EDL showed more robust growth signals and Pde2a downregulation, possibly underlying its resistance to atrophy versus soleus. Spaceflight and hindlimb unloading mice shared differential regulation of proliferation, circadian, and neuronal signaling. Shared gene regulation in muscles of humans on bedrest and space flown rodents suggest targets for mitigating muscle atrophy in space and on Earth. Spaceflight responses of C. elegans were more similar to EDL. Discrete life stages of D. melanogaster have distinct utility in anticipating EDL and soleus responses. In summary, spaceflight leads to shared and discrete molecular responses between muscle types and invertebrate models may augment mechanistic knowledge gained from rodent spaceflight and ground-based studies.

Vitamin D ameliorates adipose browning in chronic kidney disease cachexia

Patients with chronic kidney disease (CKD) are often 25(OH)D3 and 1,25(OH)2D3 insufficient. We studied whether vitamin D repletion could correct aberrant adipose tissue and muscle metabolism in a mouse model of CKD-associated cachexia. Intraperitoneal administration of 25(OH)D3 and 1,25(OH)2D3 (75 μg/kg/day and 60 ng/kg/day respectively for 6 weeks) normalized serum concentrations of 25(OH)D3 and 1,25(OH)2D3 in CKD mice. Vitamin D repletion stimulated appetite, normalized weight gain, and improved fat and lean mass content in CKD mice. Vitamin D supplementation attenuated expression of key molecules involved in adipose tissue browning and ameliorated expression of thermogenic genes in adipose tissue and skeletal muscle in CKD mice. Furthermore, repletion of vitamin D improved skeletal muscle fiber size and in vivo muscle function, normalized muscle collagen content and attenuated muscle fat infiltration as well as pathogenetic molecular pathways related to muscle mass regulation in CKD mice. RNAseq analysis was performed on the gastrocnemius muscle. Ingenuity Pathway Analysis revealed that the top 12 differentially expressed genes in CKD were correlated with impaired muscle and neuron regeneration, enhanced muscle thermogenesis and fibrosis. Importantly, vitamin D repletion normalized the expression of those 12 genes in CKD mice. Vitamin D repletion may be an effective therapeutic strategy for adipose tissue browning and muscle wasting in CKD patients.

Vitamin D repletion ameliorates adipose tissue browning and muscle wasting in infantile nephropathic cystinosis-associated cachexia

BACKGROUND

Ctns-/- mice, a mouse model of infantile nephropathic cystinosis, exhibit hypermetabolism with adipose tissue browning and profound muscle wasting. Ctns-/- mice are 25(OH)D3 and 1,25(OH)2 D3 insufficient. We investigated whether vitamin D repletion could ameliorate adipose tissue browning and muscle wasting in Ctns-/- mice.

METHODS

Twelve-month-old Ctns-/- mice and wild-type controls were treated with 25(OH)D3 and 1,25(OH)2 D3 (75 μg/kg/day and 60 ng/kg/day, respectively) or an ethylene glycol vehicle for 6 weeks. Serum chemistry and parameters of energy homeostasis were measured. We quantitated total fat mass and studied expression of molecules regulating adipose tissue browning, energy metabolism, and inflammation. We measured lean mass content, skeletal muscle fibre size, in vivo muscle function (grip strength and rotarod activity), and expression of molecules regulating muscle metabolism. We also analysed the transcriptome of skeletal muscle in Ctns-/- mice using RNAseq.

RESULTS

Supplementation of 25(OH)D3 and 1,25(OH)2 D3 normalized serum concentration of 25(OH)D3 and 1,25(OH)2 D3 in Ctns-/- mice, respectively. Repletion of vitamin D partially or fully normalized food intake, weight gain, gain of fat, and lean mass, improved energy homeostasis, and attenuated perturbations of uncoupling proteins and adenosine triphosphate content in adipose tissue and muscle in Ctns-/- mice. Vitamin D repletion attenuated elevated expression of beige adipose cell biomarkers (UCP-1, CD137, Tmem26, and Tbx1) as well as aberrant expression of molecules implicated in adipose tissue browning (Cox2, Pgf2α, and NF-κB pathway) in inguinal white adipose tissue in Ctns-/- mice. Vitamin D repletion normalized skeletal muscle fibre size and improved in vivo muscle function in Ctns-/- mice. This was accompanied by correcting the increased muscle catabolic signalling (increased protein contents of IL-1β, IL-6, and TNF-α as well as an increased gene expression of Murf-2, atrogin-1, and myostatin) and promoting the decreased muscle regeneration and myogenesis process (decreased gene expression of Igf1, Pax7, and MyoD) in skeletal muscles of Ctns-/- mice. Muscle RNAseq analysis revealed aberrant gene expression profiles associated with reduced muscle and neuron regeneration, increased energy metabolism, and fibrosis in Ctns-/- mice. Importantly, repletion of 25(OH)D3 and 1,25(OH)2 D3 normalized the top 20 differentially expressed genes in Ctns-/- mice.

CONCLUSIONS

We report the novel findings that correction of 25(OH)D3 and 1,25(OH)2 D3 insufficiency reverses cachexia and may improve quality of life by restoring muscle function in an animal model of infantile nephropathic cystinosis. Mechanistically, vitamin D repletion attenuates adipose tissue browning and muscle wasting in Ctns-/- mice via multiple cellular and molecular mechanisms.

Transcriptome response of human skeletal muscle to divergent exercise stimuli

Aerobic (AE) and resistance exercise (RE) elicit unique adaptations in skeletal muscle that have distinct implications for health and performance. The purpose of this study was to identify the unique transcriptome response of skeletal muscle to acute AE and RE. In a counterbalanced, crossover design, six healthy, recreationally active young men (27 ± 3 yr) completed acute AE (40 min of cycling, ∼70% maximal HR) and RE [8 sets, 10 reps, ∼65% 1-repetition maximum (1RM)], separated by ∼1 wk. Muscle biopsies (vastus lateralis) were obtained before and at 1 and 4 h postexercise. Whole transcriptome RNA sequencing (HiSeq2500; Illumina) was performed on cDNA synthesized from skeletal muscle RNA. Sequencing data were analyzed using HTSeq, and differential gene expression was identified using DESeq2 [adjusted P value (FDR) <0.05, >1.5-fold change from preexercise]. RE resulted in a greater number of differentially expressed genes at 1 (67 vs. 48) and 4 h (523 vs. 221) compared with AE. We identified 348 genes that were differentially expressed only following RE, whereas 48 genes were differentially expressed only following AE. Gene clustering indicated that AE targeted functions related to zinc interaction, angiogenesis, and ubiquitination, whereas RE targeted functions related to transcription regulation, cytokine activity, cell adhesion, kinase activity, and the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. ESRRG and TNFSRF12A were identified as potential targets related to the specific response of skeletal muscle to AE and RE, respectively. These data describe the early postexercise transcriptome response of skeletal muscle to acute AE and RE and further highlight that different forms of exercise stimulate unique molecular activity in skeletal muscle. NEW & NOTEWORTHY Whole transcriptome RNA sequencing was used to determine the early postexercise transcriptome response of skeletal muscle to acute aerobic (AE) and resistance exercise (RE) in untrained individuals. Although a number of shared genes were stimulated following both AE and RE, several genes were uniquely responsive to each exercise mode. These findings support the need for future research focused to better identify the role of exercise mode as it relates to targeting specific cellular skeletal muscle abnormalities.

Transcriptome of the inner circular smooth muscle of the developing mouse intestine: Evidence for regulation of visceral smooth muscle genes by the hedgehog target gene, cJun

BACKGROUND

Digestion is facilitated by coordinated contractions of the intestinal muscularis externa, a bilayered smooth muscle structure that is composed of inner circular muscles (ICM) and outer longitudinal muscles (OLM). We performed transcriptome analysis of intestinal mesenchyme tissue at E14.5, when the ICM, but not the OLM, is present, to investigate the transcriptional program of the ICM.

RESULTS

We identified 3967 genes enriched in E14.5 intestinal mesenchyme. The gene expression profiles were clustered and annotated to known muscle genes, identifying a muscle-enriched subcluster. Using publically available in situ data, 127 genes were verified as expressed in ICM. Examination of the promoter and regulatory regions for these co-expressed genes revealed enrichment for cJUN transcription factor binding sites, and cJUN protein was enriched in ICM. cJUN ChIP-seq, performed at E14.5, revealed that cJUN regulatory regions contain characteristics of muscle enhancers. Finally, we show that cJun is a target of Hedgehog (Hh), a signaling pathway known to be important in smooth muscle development, and identify a cJun genomic enhancer that is responsive to Hh.

CONCLUSIONS

This work provides the first transcriptional catalog for the developing ICM and suggests that cJun regulates gene expression in the ICM downstream of Hh signaling. Developmental Dynamics 245:614-626, 2016. © 2016 Wiley Periodicals, Inc.

Leukaemia inhibitory factor is expressed in rat gastrocnemius muscle after contusion and increases proliferation of rat L6 myoblasts via c-Myc signalling

1. Leukaemia inhibitory factor (LIF) has been shown to have an important role during muscle regeneration. The regenerative capacity of muscles after contusion injury in LIF-knockout mice is impaired compared with that of wild-type mice. 2. To clarify whether LIF modulates muscle regeneration by regulating myogenic precursor cell activity, we studied LIF expression and myogenic precursor cell activity in gastrocnemius muscles from Wistar rats at various times after contusion injury using immunohistochemistry and the direct effect of LIF on a rat myoblast cell line (L6). 3. After contusion injury, transient upregulation of the mRNA expression of LIF, LIF receptors and signal transducer and activator of transcription (STAT) 3, downstream of LIF and involved in enhanced cell proliferation, was observed. A marked increase in LIF protein in the cytosol of damaged myofibres was strongly correlated with a significant increase in the number of myogenic precursor cells (MyoD-positive cells) by 12 h after contusion. In addition, coexpression of LIF and MyoD protein in control and injured muscles after contusion injury from 3 h to 7 days was evident. 4. Treatment of L6 cells with LIF (1 ng/mL) in serum-free medium enhanced proliferation (bromodeoxyuridine incorporation) by 24 h. This was accompanied by increased expression of c-Myc protein within 12 h and was abolished by short interference RNA against c-Myc mRNA. 5. Together, the results of the present study suggest that LIF acts via paracrine and autocrine actions to regulate myogenic precursor cell activity during muscle regeneration after contusion injury and that the proliferative effect of LIF on L6 cells occurs via c-Myc signalling.

Postexercise myogenic gene expression: are human findings lost during translation?

Muscle progenitor cell (MPC) activity is exercise responsive. Post resistance-exercise alterations in myogenic messenger RNAs (mRNAs) found by us and others may initiate these events. However, these mRNA data in the absence of microscopic MPC activity data have limited this interpretation. Alternatively, with our acute exercise data as our basis, we propose that these genes may control other hypertrophic processes in postmitotic fibers.

Satellite cell activity in muscle regeneration after contusion in rats

1. The role of satellite cells in muscle growth during development is well documented, but the involvement of these cells in muscle repair after contusion is less well known. In the present study, we investigated the time-course of satellite cell activity (from 3h to 7days) after contusion of rat gastrocnemius muscle using specific molecular markers for immunofluorescence and real-time polymerase chain reaction (PCR). 2. Inflammation of the injured muscle occurred within 6h, followed by disintegration of the damaged myofibres within 12h. Newly formed myofibres appeared by Day 7. 3. The number of MyoD-positive nuclei (activated satellite cells) in the injured muscle was significantly increased by 6h, reaching a maximum by 12h after contusion. However, the number of MyoD-positive nuclei decreased towards control levels by Day 7. Changes in the number of bromodeoxyuridine-labelled nuclei (proliferating satellite cells) paralleled the changes seen in the number of MyoD-positive nuclei. Conversely, expression of myogenin protein was not apparent until Day 3 and increased further by Day 7. Colabelling of MyoD and myogenin was seen in only a few cells. 4. The time-course of MyoD mRNA expression corresponded with MyoD protein expression. However, there were two peaks in myogenin mRNA expression: 6h and Day 7 after contusion. The second peak coincided with upregulation of myostatin mRNA levels. 5. The results of the present study suggest that contusion activates a homogeneous population of satellite cells to proliferate within 3days, followed by differentiation to form new myofibres. The latter may be regulated, in part, by myostatin.

Regulation of muscle satellite cell activation and chemotaxis by angiotensin II

The role of angiotensin II (Ang II) in skeletal muscle is poorly understood. We report that pharmacological inhibition of Ang II signaling or ablation of the AT1a receptor significantly impaired skeletal muscle growth following myotrauma, in vivo, likely due to impaired satellite cell activation and chemotaxis. In vitro experiments demonstrated that Ang II treatment activated quiescent myoblasts as evidenced by the upregulation of myogenic regulatory factors, increased number of β-gal+, Myf5-LacZ myoblasts and the acquisition of cellular motility. Furthermore, exogenous treatment with Ang II significantly increased the chemotactic capacity of C2C12 and primary cells while AT1a^−/− myoblasts demonstrated a severe impairment in basal migration and were not responsive to Ang II treatment. Additionally, Ang II interacted with myoblasts in a paracrine-mediated fashion as 4 h of cyclic mechanical stimulation resulted in Ang II-induced migration of cocultured myoblasts. Ang II-induced chemotaxis appeared to be regulated by multiple mechanisms including reorganization of the actin cytoskeleton and augmentation of MMP2 activity. Collectively, these results highlight a novel role for Ang II and ACE inhibitors in the regulation of skeletal muscle growth and satellite cell function.

The time-course analysis of gene expression during wound healing in mouse skin

RNA analysis has been applied to forensic work to determine wound age. We investigated mRNA expression using quantitative RT-PCR of ten genes, including c-fos, fosB, mitogen-activated protein kinase phosphatase-1 (MKP-1), CD14, chemokine (C-C motif) ligand 9 (CCL9), placenta growth factor (PlGF), mast cell protease-5 (MCP-5), growth arrest specific 5 (Gas5), beta-2 microglobulin (B2M) and major urinary protein-1 (MUP-1), in terms of repair response in adult mice. The expression level of c-fos, fosB and MKP-1 transcripts increased drastically, peaked within 1h, and that of the CD14 and CCL9 transcripts peaked from 12 to 24h. An increase in PlGF and MCP-5 mRNA appeared on about day 5. Gas5, B2M and MUP-1 transcripts showed no significant change. Each gene had differentially expressional patterns with time-course. Our result implied that the observation of the 7 genes in wounded skin could serve to aid in the accurate diagnosis of wound age.

Midkine and its receptor in regenerating rat skeletal muscle after bupivacaine injection

Midkine (MK) is a multifunctional cytokine and heparin-binding growth factor with neurotrophic activity. MK and its receptor were examined for up to 14 days in a chemically injured rat muscle regeneration process caused by the injection of bupivacaine using immunohistochemical and Western blot analysis. Although MK immunoreactivity was not detectable in the mature uninjured skeletal muscle, MK was strongly detected in the regenerating muscle cells. MK immunoreactivity was observed in the myoblast-like cells and myotubes, which were desmin-positive cells, whereas it was not detectable in the surviving normal muscle fibers. Most myotubes labeling for desmin showed MK immunoreactivity 5-7days after the injury. However, MK immunoreactivity was not detected 14 days after the injury. Immunoreactivity of low-density lipoprotein receptor-related protein (LRP), a cell membrane receptor of MK, was detected in the regenerating muscle cells, whereas it was not detected in the normal adult skeletal muscle and surviving muscle. These findings suggested that MK was involved. MK may have a role for differentiation during skeletal muscle regeneration and may be taken up in an autocrine fashion with LRP.

Signaling satellite-cell activation in skeletal muscle: markers, models, stretch, and potential alternate pathways

Activation of skeletal muscle satellite cells, defined as entry to the cell cycle from a quiescent state, is essential for normal growth and for regeneration of tissue damaged by injury or disease. This review focuses on early events of activation by signaling through nitric oxide and hepatocyte growth factor, and by mechanical stimuli. The impact of various model systems used to study activation and the regulation of satellite-cell quiescence are placed in the context of activation events in other tissues, concluding with a speculative model of alternate pathways signaling satellite-cell activation.

Expression of connexins during differentiation and regeneration of skeletal muscle: functional relevance of connexin43

The molecular mechanisms regulating skeletal muscle regeneration and differentiation are not well understood. We analyzed the expression of connexins (Cxs) 40, 43 and 45 in normal and regenerating tibialis anterior muscle and in primary cultures of differentiating myoblasts in adult and newborn mice, respectively. Cxs 45 and 43, but not 40, were strongly expressed in normal muscle and their expression was upregulated during regeneration. Furthermore, the functional role of Cx43 during differentiation and regeneration was examined after induced deletion of Cx43 in transgenic mice. In vivo, the inducible deletion of Cx43 delayed the formation of myofibers and prolonged the expression of myogenin during regeneration. In primary cultures of satellite cell-derived myoblasts, induced deletion of Cx43 led to decreased expression of myogenin and MyoD, dye coupling, creatine kinase activity and myoblast fusion. Thus, the expression of Cx45 and Cx43 is upregulated during skeletal muscle regeneration and Cx43 is required for normal myogenesis in vitro and adult muscle regeneration in vivo.

Signals from damaged but not undamaged skeletal muscle induce myogenic differentiation of rat bone-marrow-derived mesenchymal stem cells

The regenerative capacity of skeletal muscle has been usually attributed to resident satellite cells, which, upon activation by local or distant stimuli, initiate a myogenic differentiation program. Although recent studies have revealed that bone-marrow-derived progenitor cells may also participate in regenerative myogenesis, the signals and mechanisms involved in this process have not been elucidated. This study was designed to investigate whether signals from injured rat skeletal muscle were competent to induce a program of myogenic differentiation in expanded cultures of rat bone-marrow-derived mesenchymal stem cells (MSC). We observed that the incubation of MSC with a conditioned medium prepared from chemically damaged but not undamaged muscle resulted in a time-dependent change from fibroblast-like into elongated multinucleated cells, a transient increase in the number of MyoD positive cells, and the subsequent onset of myogenin, alpha-actinin, and myosin heavy chain expression. These results show that damaged rat skeletal muscle is endowed with the capacity to induce myogenic differentiation of bone-marrow-derived mesenchymal progenitors.

Satellite cell activation on fibers: modeling events in vivo — an invited review

Knowledge of the events underlying satellite cell activation and the counterpart maintenance of quiescence is essential for planning therapies that will promote the growth and regeneration of skeletal muscle in healthy, disease and aging. By modeling those events of satellite cell activation in studies of single muscle fibers or muscles in culture, the roles of mechanical stretching and nitric oxide are becoming understood. Recent studies demonstrated that stretch-induced activation is very rapid and exhibits some features of satellite cell heterogeneity. As well, gene expression studies showed that expression of the c-met receptor gene rises rapidly after stretching muscles in culture compared to those without stretch. This change in gene expression during activation, and the maintenance of quiescence in both normal and dystrophic muscles are dependent on NO, as they are blocked by inhibition of nitric oxide synthase (NOS). Mechanical, contractile activity is the defining feature of muscle function. Therefore, ongoing studies of stretch effects in satellite cell activation and quiescence in quiescent fiber and muscle cultures provides appropriate models by which to explore the regulatory steps in muscle in vivo under many conditions related to disease, repair, rehabilitation, growth and the prevention or treatment of atrophy.Key words: regeneration, stretch, myofiber culture, muscular dystrophy, quiescence.

Expression and neural control of follistatin versus myostatin genes during regeneration of mouse soleus

Follistatin and myostatin are two secreted proteins involved in the control of muscle mass during development. These two proteins have opposite effects on muscle growth, as documented by genetic models. The aims of this work were to analyze in mouse, by using in situ hybridization, the spatial and temporal expression patterns of follistatin and myostatin mRNAs during soleus regeneration after cardiotoxin injury, and to investigate the influence of innervation on the accumulation of these two transcripts. Follistatin transcripts could be detected in activated satellite cells as early as the first stages of regeneration and were transiently expressed in forming myotubes. In contrast, myostatin mRNAs accumulated persistently throughout the regeneration process as well as in adult control soleus. Denervation significantly affected both follistatin and myostatin transcript accumulation, but in opposite ways. Muscle denervation persistently reduced the levels of myostatin transcripts as early as the young myotube stage, whereas the levels of follistatin mRNA were strongly increased in the small myotubes in the late stages of regeneration. These results are discussed with regard to the potential functions of both follistatin, as a positive regulator of muscle differentiation, and myostatin, as a negative regulator of skeletal muscle growth. We suggest that the belated up-regulation of the follistatin mRNA level in the small myotubes of the regenerating soleus as well as the down-regulation of the myostatin transcript level after denervation contribute to the differentiation process in denervated regenerating muscle.

Gene expression patterns in skeletal muscle of thermally injured children treated with oxandrolone

OBJECTIVE

To analyze gene expression patterns in skeletal muscle from burned children.

SUMMARY BACKGROUND DATA

Analysis of gene expression patterns in skeletal muscle from burned children can help provide a fundamental understanding of muscle wasting at the molecular level. This study is the first to use such an approach in burned children receiving anabolic treatment.

METHODS

Children who received 0.1 mg/kg oxandrolone twice a day (n = 7) were compared to placebo (n = 7). Net protein balance was determined before and after treatment with oxandrolone. Total RNA, extracted from muscle biopsies obtained from burned children age 3 to 18 years, was purified, reverse transcribed, and biotinylated cRNA hybridized to the human high-density oligonucleotide array (U95Av2). Western blot analysis verified the mRNA changes at their protein level.

RESULTS

DNA microarray analysis showed two genes significantly changed in muscle from burned children receiving placebo, while the expression of 21 genes was altered with oxandrolone. Muscle net protein balance increased with oxandrolone treatment compared to placebo.

CONCLUSIONS

DNA microarray technology will help identify molecular changes that can serve as targets for new therapies to attenuate muscle wasting in severely burned children and thus improve recovery and early rehabilitation.

Down-regulation of an ankyrin repeat-containing protein, V-1, during skeletal muscle differentiation and its re-expression in the regenerative process of muscular dystrophy

Using Western blot analysis and immunohistochemical methods, we examined the expression of V-1, a member of the ankyrin repeat-containing protein family, during differentiation and regeneration of skeletal muscle. The expression of V-1 was high in cultured myoblasts and decreased during their differentiation into myotubes, while high expression was maintained when muscle differentiation was inhibited by treatment with basic fibroblast growth factor. Down-regulation of V-1 also occurred during in vivo muscle differentiation from embryonic to postnatal stages, reaching an undetectable level in mature skeletal muscle. In contrast, strong V-1 immunoreactivity was detected again in myoblasts and regenerating muscle fibers with a small diameter, which were observed in Duchenne muscular dystrophy and its animal model, mdx mouse. Thus, it seems that V-1 is a good marker for early stage of muscle regeneration and changes of its expression suggest that V-1 plays a role in prenatal muscle differentiation and postnatal muscle regeneration.

In vivo activation of STAT3 signaling in satellite cells and myofibers in regenerating rat skeletal muscles

Although growth factors and cytokines play critical roles in skeletal muscle regeneration, intracellular signaling molecules that are activated by these factors in regenerating muscles have been not elucidated. Several lines of evidence suggest that leukemia inhibitory factor (LIF) is an important cytokine for the proliferation and survival of myoblasts in vitro and acceleration of skeletal muscle regeneration. To elucidate the role of LIF signaling in regenerative responses of skeletal muscles, we examined the spatial and temporal activation patterns of an LIF-associated signaling molecule, the signal transducer and activator transcription 3 (STAT3) proteins in regenerating rat skeletal muscles induced by crush injury. At the early stage of regeneration, activated STAT3 proteins were first detected in the nuclei of activated satellite cells and then continued to be activated in proliferating myoblasts expressing both PCNA and MyoD proteins. When muscle regeneration progressed, STAT3 signaling was no longer activated in differentiated myoblasts and myotubes. In addition, activation of STAT3 was also detected in myonuclei within intact sarcolemmas of surviving myofibers that did not show signs of necrosis. These findings suggest that activation of STAT3 signaling is an important molecular event that induces the successful regeneration of injured skeletal muscles.

MRF4 protein expression in regenerating rat muscle

The myogenic transcription factors of the MyoD family are not expressed in normal adult skeletal muscle. They are upregulated at the transcript and protein levels in a precisely coordinated manner during regeneration. While the cellular distribution of MyoD, myf-5, and myogenin expression in regenerating muscle is well documented, little is known about the exact localization of MRF4. It was the aim of this study to monitor the cellular distribution of MRF4 protein during regeneration. The soleus muscle of 6-week-old male Wistar rats was devascularized and allowed to regenerate for 2, 5, 10 or 14 days. Immunostaining revealed the presence of MRF4 throughout the time periods studied. Expression was detected in the nuclei of myofibers which had survived the devascularization procedure 2 days after necrosis was induced. In nuclei of newly formed myotubes and young myofibers, MRF4 was co-expressed with MyoD and myogenin. MRF4 protein was absent from satellite cells (SC), with anti-M-cadherin being used as a SC marker. Taken together, our results demonstrate that MRF4 protein expression in regenerated fibers is restricted to the time around and after fusion. The absence of MRF4 protein in SC suggests that the role of MRF4 during regeneration is distinct from myf-5, MyoD, and myogenin.

Expression and neural control of myogenic regulatory factor genes during regeneration of mouse soleus

Given the importance of the myogenic regulatory factors (MRFs) for myoblast differentiation during development, the aims of this work were to clarify the spatial and temporal expression pattern of the four MRF mRNAs during soleus regeneration in mouse after cardiotoxin injury, using in situ hybridization, and to investigate the influence of innervation on the expression of each MRF during a complete degeneration/regeneration process. For this, we performed cardiotoxin injury-induced regeneration experiments on denervated soleus muscle. Myf-5, MyoD, and MRF4 mRNAs were detected in satellite cell-derived myoblasts in the first stages of muscle regeneration analyzed (2--3 days P-I). The Myf-5 transcript level dramatically decreased in young multinucleated myotubes, whereas MyoD and MRF4 transcripts were expressed persistently throughout the regeneration process. Myogenin mRNA was transiently expressed in forming myotubes. These results are discussed with regard to the potential relationships between MyoD and MRF4 in the satellite cell differentiation pathway. Muscle denervation precociously (at 8 days P-I) upregulated both the Myf-5 and the MRF4 mRNA levels, whereas the increase of both MyoD and myogenin mRNA levels was observed later, in the late stages of regeneration (30 days P-I). This significant accumulation of each differentially upregulated MRF during soleus regeneration after denervation suggests that each myogenic factor might have a distinct role in the regulatory control of muscle gene expression. This role is discussed in relation to the expression of the nerve-regulated genes, such as the nAChR subunit gene family. (J Histochem Cytochem 49:887-899, 2001)

Effect of chronic denervation and denervation-reinnervation on cytoplasmic creatine kinase transcript accumulation

The extensor digitorum longus (EDL) and soleus muscles of adult mice were chronically denervated or denervated and allowed to reinnervate. Muscles were evaluated 1, 5, 14, 21, and 52 days after sciaticectomy. In terms of weight loss, myofiber atrophy, degeneration, and fibrosis, the soleus muscle was more affected than the EDL by chronic denervation. Fifty-two days after chronic denervation, the number of molecules of MCK/ng total RNA in both muscles (determined with competitive PCR) decreased, with the soleus muscle being more affected. At that stage, BCK mRNA levels in the denervated soleus were unchanged, but they were increased (>50%) in the EDL. Reinnervation restored MCK transcript accumulation in the EDL, whereas, in the soleus MCK, transcripts exceeded control values by 57%, approaching levels in the reinnervated EDL. Despite restoration of MCK mRNA levels, the number of molecules of BCK mRNA/ng total RNA was four- to fivefold higher in reinnervated versus control muscles, suggesting that the genes encoding the CK mRNAs are not coordinately regulated in adult muscle. The role of denervation induced, fiber type changes in regulating CK mRNA accumulation has been evaluated. Electron microscopic analyses have established that fibrosis is not a factor that determines BCK mRNA levels in the chronically denervated or denervated-reinnervated muscles. CK isozyme analyses support the hypothesis that a greater proportion of BCK mRNA found in 52 day chronically denervated and denervated-reinnervated muscles is produced in myofibers vs. nonmuscle cells than in control muscles.

Morphological and functional recovery from diaphragm injury: an in vivo rat diaphragm injury model

Our objective was to develop an in vivo model to study the timing and mechanisms underlying diaphragm injury and repair. Diaphragm injury was induced in anesthetized rats by the application of a 100 mM caffeine solution for a 10-min period to the right abdominal diaphragm surface. Diaphragms were removed 1, 4, 6, 12, 24, 48, 72, and 96 h and 10 days after the injury, with contractile function being assessed in strips in vitro by force-frequency curves. The extent of caffeine-induced membrane injury was indicated by the percentage of fibers with a fluorescent cytoplasm revealed by inward leakage of the procion orange dye. One hour after caffeine exposure, 32.9 ± 3.1 (SE) % of fibers showed membrane injury that resulted in 70% loss of muscle force. Within 72–96 h, the percentage of fluorescent cells decreased to control values. Muscle force, however, was still reduced by 30%. Complete muscle strength recovery was observed 10 days after the injury. Whereas diaphragmatic fiber repair occurred within 4 days after injury induction, force recovery took up to 10 days. We suggest that the caffeine-damaged rat diaphragm is a useful model to study the timing and mechanisms of muscle injury and repair.

Gene expression of receptors for IL-6, LIF, and CNTF in regenerating skeletal muscles

The biological actions of interleukin-6 (IL-6), leukemia inhibitory factor (LIF), and ciliary neurotrophic factor (CNTF) are mediated via respective functional receptor complexes consisting of a common signal-transducing component, gp130, and other specific receptor components, IL-6 receptor alpha (IL-6R), LIF receptor beta (LIFR), and CNTF receptor alpha (CNTFR). IL-6, LIF, and CNTF are implicated in skeletal muscle regeneration. However, the cell populations that express these receptor components in regenerating muscles are unknown. Using in situ hybridization histochemistry, we examined spatiotemporal expression patterns of gp130, IL-6R, LIFR, and CNTFR mRNAs in regenerating muscles after muscle contusion. At the early stages of regeneration (from 3 hr to Day 2 post contusion), significant signals for gp130 and LIFR mRNAs were detected in myonuclei and/or nuclei of muscle precursor cells (mpcs) and in mononuclear cells located in extracellular spaces between myofibers after muscle contusion, but IL-6R mRNA was expressed only in mononuclear cells. At Day 7 post contusion, signals for gp130, LIFR, and IL-6R mRNAs were not detected in newly formed myotubes, whereas the CNTFR mRNA level was upregulated in myotubes. These findings suggest that the upregulation of receptor subunits in distinct cell populations plays an important role in the effective regeneration of both myofibers and motor neurons. (J Histochem Cytochem 48:1203-1213, 2000)

A role for nitric oxide in muscle repair: nitric oxide-mediated activation of muscle satellite cells

Muscle satellite cells are quiescent precursors interposed between myofibers and a sheath of external lamina. Although their activation and recruitment to cycle enable muscle repair and adaptation, the activation signal is not known. Evidence is presented that nitric oxide (NO) mediates satellite cell activation, including morphological hypertrophy and decreased adhesion in the fiber-lamina complex. Activation in vivo occurred within 1 min after injury. Cell isolation and histology showed that pharmacological inhibition of nitric oxide synthase (NOS) activity prevented the immediate injury-induced myogenic cell release and delayed the hypertrophy of satellite cells in that muscle. Transient activation of satellite cells in contralateral muscles 10 min later suggested that a circulating factor may interact with NO-mediated signaling. Interestingly, satellite cell activation in muscles of mdx dystrophic mice and NOS-I knockout mice quantitatively resembled NOS-inhibited release of normal cells, in agreement with reports of displaced and reduced NOS expression in dystrophin-deficient mdx muscle and the complete loss of NOS-I expression in knockout mice. Brief NOS inhibition in normal and mdx mice during injury produced subtle alterations in subsequent repair, including apoptosis in myotube nuclei and myotube formation inside laminar sheaths. Longer NOS inhibition delayed and restricted the extent of repair and resulted in fiber branching. A model proposes the hypothesis that NO release mediates satellite cell activation, possibly via shear-induced rapid increases in NOS activity that produce "NO transients."

Neural and hormonal control of expression of myogenic regulatory factor genes during regeneration of Xenopus fast muscles: myogenin and MRF4 mRNA accumulation are neurally regulated oppositely

With the aim to investigate the influence of both innervation and thyroid hormone, on the expression of the MRFs during muscle regeneration, we performed cardiotoxin injury-induced regeneration experiments on fast muscles of adult Xenopus laevis subjected to different experimental conditions, including denervation and T3 treatment, and analyzed the accumulation of the four myogenic regulatory factors (MRFs) using RT-PCR and in situ hybridization. We show here that manipulation of hormone levels or innervation resulted in differential alterations of MRF expression. Denervation and T3 treatment transiently down-regulated Myf-5 mRNA levels at the beginning of the regeneration process. Myf-5 was the only myogenic factor subject to thyroid hormone influence. Muscle denervation persistently reduces the levels of MRF4 transcripts as early as the first stages of regeneration, whereas the levels of myogenin mRNA were increased in the late stages of regeneration. This suggests that MRF4 expression may be induced by innervation and hence may be involved in mediating transcriptional responses to innervation and that myogenin expression may compensate for the down-regulation of MRF4 gene. This switch in MRF gene expression following denervation could have important consequences for the ability of Xenopus regenerating muscles to recover function after denervation.

Leukemia inhibitory factor, glial cell line-derived neurotrophic factor, and their receptor expressions following muscle crush injury

Using in situ hybridization histochemistry, we characterized the spatiotemporal gene expression patterns of leukemia inhibitory factor (LIF) and glial cell line-derived neurotrophic factor (GDNF), and their receptor components (LIFR, GFR-alpha1, RET) induced in muscle cells, intramuscular nerves, and motoneurons in the regeneration processes of both muscle cells and nerves following muscle contusion. Muscle contusion induced upregulation of GDNF and GFR-alpha1 mRNAs in Schwann cell-like cells in the intramuscular nerves and of LIFR mRNA in damaged muscle cells. LIFR, GFR-alpha1, and RET mRNA expressions in motoneurons were upregulated following muscle contusion. Muscle contusion also induced more rapid, prominent transactivations of GFR-alpha1 and RET genes in motoneurons than did sciatic nerve axotomy. These findings suggest that rapid and prominent upregulation of the receptor components for LIF and GDNF in motoneurons is important for the regeneration of intramuscular motor nerves damaged by muscle contusion.

The transition from proliferation to differentiation is delayed in satellite cells from mice lacking MyoD

Satellite cells from adult rat muscle coexpress proliferating cell nuclear antigen and MyoD upon entry into the cell cycle, suggesting that MyoD plays a role during the recruitment of satellite cells. Moreover, the finding that muscle regeneration is compromised in MyoD-/- mice, has provided evidence for the role of MyoD during myogenesis in adult muscle. In order to gain further insight into the role of MyoD during myogenesis in the adult, we compared satellite cells from MyoD-/- and wildtype mice as they progress through myogenesis in single-myofiber cultures and in tissue-dissociated cell cultures (primary cultures). Satellite cells undergoing proliferation and differentiation were traced immunohistochemically using antibodies against various regulatory proteins. In addition, an antibody against the mitogen-activated protein kinases ERK1 and ERK2 was used to localize the cytoplasm of the fiber-associated satellite cells regardless of their ability to express specific myogenic regulatory factor proteins. We show that during the initial days in culture the myofibers isolated from both the MyoD-/- and the wildtype mice contain the same number of proliferating, ERK+ satellite cells. However, the MyoD-/- satellite cells continue to proliferate and only a very small number of cells transit into the myogenin+ state, whereas the wildtype cells exit the proliferative compartment and enter the myogenin+ stage. Analyzing tissue-dissociated cultures of MyoD-/- satellite cells, we identified numerous cells whose nuclei were positive for the Myf5 protein. In contrast, quantification of Myf5+ cells in the wildtype cultures was difficult due to the low level of Myf5 protein present. The Myf5+ cells in the MyoD-/- cultures were often positive for desmin, similar to the MyoD+ cells in the wildtype cultures. Myogenin+ cells were identified in the MyoD-/- primary cultures, but their appearance was delayed compared to the wildtype cells. These "delayed" myogenin+ cells can express other differentiation markers such as MEF2A and cyclin D3 and fuse into myotubes. Taken together, our studies suggest that the presence of MyoD is critical for the normal progression of satellite cells into the myogenin+, differentiative state. It is further proposed that the Myf5+/MyoD- phenotype may represent the myogenic stem cell compartment which is capable of maintaining the myogenic precursor pool in the adult muscle.

Fibroblast growth factor promotes recruitment of skeletal muscle satellite cells in young and old rats

Although the role of satellite cells in muscle growth and repair is well recognized, understanding of the molecular events that accompany their activation and proliferation is limited. In this study, we used the single myofiber culture model for comparing the proliferative dynamics of satellite cells from growing (3-week-old), young adult (8- to 10-week-old), and old (9- to 11-month-old) rats. In these fiber cultures, the satellite cells are maintained in their in situ position underneath the fiber basement membrane. We first demonstrate that the cytoplasm of fiber-associated satellite cells can be monitored with an antibody against the extracellular signal regulated kinases 1 and 2 (ERK1 and ERK2), which belong to the mitogen-activated protein kinase (MAPK) superfamily. With this immunocytological marker, we show that the satellite cells from all three age groups first proliferate and express PCNA and MyoD, and subsequently, about 24 hr later, exit the PCNA+/MyoD+ state and become positive for myogenin. For all three age groups, fibroblast growth factor 2 (FGF2) enhances by about twofold the number of satellite cells that are capable of proliferation, as determined by monitoring the number of cells that transit from the MAPK+ phenotype to the PCNA+/MAPK+ or MyoD+/MAPK+ phenotype. Furthermore, contrary to the commonly accepted convention, we show that in the fiber cultures FGF2 does not suppress the subsequent transition of the proliferating cells into the myogenin+ compartment. Although myogenesis of satellite cells from growing, young adult, and old rats follows a similar program, two distinctive features were identified for satellite cells in fiber cultures from the old rats. First, a large number of MAPK+ cells do not appear to enter the MyoD-myogenin expression program. Second, the maximal number of proliferating satellite cells is attained a day later than in cultures from the young adults. This apparent "lag" in proliferation was not affected by hepatocyte growth factor (HGF), which has been implicated in accelerating the first round of satellite cell proliferation. HGF and FGF2 were equally efficient in promoting proliferation of satellite cells in fibers from old rats. Collectively, the investigation suggests that FGF plays a critical role in the recruitment of satellite cells into proliferation.

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.

Localization of leukemia inhibitory factor and interleukin-6 messenger ribonucleic acids in regenerating rat skeletal muscle

In the present study, we characterized both temporal and spatial expression patterns of leukemia inhibitory factor (LIF) and interleukin-6 (IL-6) messenger ribonucleic acids (mRNAs) in injured skeletal muscle using in situ hybridization. LIF and IL-6 mRNAs were expressed in mononucleated cells and damaged muscle cells. Further, signals for LIF mRNA were also detected in Schwann cell-like cells of intramuscular nerves. These results suggest that the earliest events involved in the repair of injured muscles and nerves may be triggered by these cytokines.

Localization of Myf-5, MRF4 and alpha cardiac actin mRNAs in regenerating Xenopus skeletal muscle

We have analysed the spatial and temporal expression patterns of Myf-5, MRF4 and alpha cardiac actin mRNAs during muscle regeneration following cardiotoxin injury in adult Xenopus laevis using in situ hybridization. Myf-5 transcripts began to be detected in the activated satellite cells as early as the beginning of the regeneration process, then dramatically decreased in young plurinucleated myotubes. MRF4 mRNA was detected later, just before the young myotube stage, and was strongly expressed during the different stages of the maturation of myotubes. Like Myf-5, alpha cardiac actin mRNA began to accumulate early in activated satellite cells. These results, which contribute to an overview of the expression of the genes coding for myogenic bHLH proteins during muscle regeneration, are discussed in relation to the expression of these factors during development.

Analysis of MyoD, myogenin, and muscle-specific gene mRNAs in regenerating Xenopus skeletal muscle

We have analyzed in adult Xenopus laevis, using in situ hybridization, the spatial and temporal expression patterns of MyoD, myogenin, and alpha-skeletal actin and fast myosin heavy chain mRNAs during muscle regeneration following cardiotoxin injury. MyoD transcripts could be detected in the satellite cells as early as the first stage of regeneration and were expressed persistently throughout the regeneration process. Myogenin mRNAs were transiently expressed in forming myotubes. alpha-Skeletal actin and fast myosin heavy chain mRNAs were detected precociously, before the young myotube stage. This work has shown, for the first time, the presence of myogenin transcripts during Xenopus myogenesis.