Role(s) of nucleoli and phosphorylation of ribosomal protein S6 and/or HSP27 in the regulation of muscle mass

The Effect of SERCA Activation on Functional Characteristics and Signaling of Rat Soleus Muscle upon 7 Days of Unloading

Skeletal muscle abnormalities and atrophy during unloading are accompanied by the accumulation of excess calcium in the sarcoplasm. We hypothesized that calcium accumulation may occur, among other mechanisms, due to the inhibition of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) activity. Consequently, the use of the SERCA activator will reduce the level of calcium in the sarcoplasm and prevent the negative consequences of muscle unloading. Wistar rats were randomly assigned into one of three groups (eight rats per group): control rats with placebo (C), 7 days of unloading/hindlimb suspension with placebo (7HS), and 7 days of unloading treated with SERCA activator CDN1163 (7HSC). After seven days of unloading the soleus muscle, the 7HS group displayed increased fatigue in the ex vivo test, a significant increase in the level of calcium-dependent CaMK II phosphorylation and the level of tropomyosin oxidation, as well as a decrease in the content of mitochondrial DNA and protein, slow-type myosin mRNA, and the percentage of slow-type muscle fibers. All of these changes were prevented in the 7HSC group. Moreover, treatment with CDN1163 blocked a decrease in the phosphorylation of p70S6k, an increase in eEF2 phosphorylation, and an increase in MuRF-1 mRNA expression. Nevertheless, there were no differences in the degree of fast and slow muscle fiber atrophy between the 7HS and 7HSC groups. Conclusion: SERCA activation during 7 days of unloading prevented an increase in soleus fatigue, the decrease of slow-type myosin, mitochondrial markers, and markers of calcium homeostasis but had no effect on muscle atrophy.

ATP spreads inflammation to other limbs through crosstalk between sensory neurons and interneurons

Neural circuits between lesions are one mechanism through which local inflammation spreads to remote positions. Here, we show the inflammatory signal on one side of the joint is spread to the other side via sensory neuron-interneuron crosstalk, with ATP at the core. Surgical ablation or pharmacological inhibition of this neural pathway prevented inflammation development on the other side. Mechanistic analysis showed that ATP serves as both a neurotransmitter and an inflammation enhancer, thus acting as an intermediary between the local inflammation and neural pathway that induces inflammation on the other side. These results suggest blockade of this neural pathway, which is named the remote inflammation gateway reflex, may have therapeutic value for inflammatory diseases, particularly those, such as rheumatoid arthritis, in which inflammation spreads to remote positions.

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.

Muscle unloading: A comparison between spaceflight and ground-based models

Prolonged unloading of skeletal muscle, a common outcome of events such as spaceflight, bed rest and hindlimb unloading, can result in extensive metabolic, structural and functional changes in muscle fibres. With advancement in investigations of cellular and molecular mechanisms, understanding of disuse muscle atrophy has significantly increased. However, substantial gaps exist in our understanding of the processes dictating muscle plasticity during unloading, which prevent us from developing effective interventions to combat muscle loss. This review aims to update the status of knowledge and underlying mechanisms leading to cellular and molecular changes in skeletal muscle during unloading. We have also discussed advances in the understanding of contractile dysfunction during spaceflights and in ground-based models of muscle unloading. Additionally, we have elaborated on potential therapeutic interventions that show promising results in boosting muscle mass and strength during mechanical unloading. Finally, we have identified key gaps in our knowledge as well as possible research direction for the future.

Neural activity regulates autoimmune diseases through the gateway reflex

The brain, spinal cord and retina are protected from blood-borne compounds by the blood-brain barrier (BBB), blood-spinal cord barrier (BSCB) and blood-retina barrier (BRB) respectively, which create a physical interface that tightly controls molecular and cellular transport. The mechanical and functional integrity of these unique structures between blood vessels and nervous tissues is critical for maintaining organ homeostasis. To preserve the stability of these barriers, interplay between constituent barrier cells, such as vascular endothelial cells, pericytes, glial cells and neurons, is required. When any of these cells are defective, the barrier can fail, allowing blood-borne compounds to encroach neural tissues and cause neuropathologies. Autoimmune diseases of the central nervous system (CNS) and retina are characterized by barrier disruption and the infiltration of activated immune cells. Here we review our recent findings on the role of neural activity in the regulation of these barriers at the vascular endothelial cell level in the promotion of or protection against the development of autoimmune diseases. We suggest nervous system reflexes, which we named gateway reflexes, are fundamentally involved in these diseases. Although their reflex arcs are not completely understood, we identified the activation of specific sensory neurons or receptor cells to which barrier endothelial cells respond as effectors that regulate gateways for immune cells to enter the nervous tissue. We explain this novel mechanism and describe its role in neuroinflammatory conditions, including models of multiple sclerosis and posterior autoimmune uveitis.

AMP-Activated Protein Kinase as a Key Trigger for the Disuse-Induced Skeletal Muscle Remodeling

Molecular mechanisms that trigger disuse-induced postural muscle atrophy as well as myosin phenotype transformations are poorly studied. This review will summarize the impact of 5′ adenosine monophosphate -activated protein kinase (AMPK) activity on mammalian target of rapamycin complex 1 (mTORC1)-signaling, nuclear-cytoplasmic traffic of class IIa histone deacetylases (HDAC), and myosin heavy chain gene expression in mammalian postural muscles (mainly, soleus muscle) under disuse conditions, i.e., withdrawal of weight-bearing from ankle extensors. Based on the current literature and the authors’ own experimental data, the present review points out that AMPK plays a key role in the regulation of signaling pathways that determine metabolic, structural, and functional alternations in skeletal muscle fibers under disuse.

Proteomic analyses of brain tumor cell lines amidst the unfolded protein response

Brain tumors such as high grade gliomas are among the deadliest forms of human cancers. The tumor environment is subject to a number of cellular stressors such as hypoxia and glucose deprivation. The persistence of the stressors activates the unfolded proteins response (UPR) and results in global alterations in transcriptional and translational activity of the cell. Although the UPR is known to effect tumorigenesis in some epithelial cancers, relatively little is known about the role of the UPR in brain tumors. Here, we evaluated the changes at the molecular level under homeostatic and stress conditions in two glioma cell lines of differing tumor grade. Using mass spectrometry analysis, we identified proteins unique to each condition (unstressed/stressed) and within each cell line (U87MG and UPN933). Comparing the two, we find differences between both the conditions and cell lines indicating a unique profile for each. Finally, we used our proteomic data to identify the predominant pathways within these cells under unstressed and stressed conditions. Numerous predominant pathways are the same in both cell lines, but there are differences in biological and molecular classifications of the identified proteins, including signaling mechanisms, following UPR induction; we see that relatively minimal proteomic alterations can lead to signaling changes that ultimately promote cell survival.

Starring or Supporting Role? Satellite Cells and Skeletal Muscle Fiber Size Regulation

Recent loss-of-function studies show that satellite cell depletion does not promote sarcopenia or unloading-induced atrophy, and does not prevent regrowth. Although overload-induced muscle fiber hypertrophy is normally associated with satellite cell-mediated myonuclear accretion, hypertrophic adaptation proceeds in the absence of satellite cells in fully grown adult mice, but not in young growing mice. Emerging evidence also indicates that satellite cells play an important role in remodeling the extracellular matrix during hypertrophy.

The effects of dietary β-guanidinopropionic acid on growth and muscle fiber development in juvenile red porgy, Pagrus pagrus

β-guanidinopropionic acid (β-GPA) has been used in mammalian models to reduce intracellular phosphocreatine (PCr) concentration, which in turn lowers the energetic state of cells. This leads to changes in signaling pathways that attempt to re-establish energetic homeostasis. Changes in those pathways elicit effects similar to those of exercise such as changes in body and muscle growth, metabolism, endurance and health. Generally, exercise effects are beneficial to fish health and aquaculture, but inducing exercise in fishes can be impractical. Therefore, this study evaluated the potential use of supplemental β-GPA to induce exercise-like effects in a rapidly growing juvenile teleost, the red porgy (Pagrus pagrus). We demonstrate for the first time that β-GPA can be transported into teleost muscle fibers and is phosphorylated, and that this perturbs the intracellular energetic state of the cells, although to a lesser degree than typically seen in mammals. β-GPA did not affect whole animal growth, nor did it influence skeletal muscle fiber size or myonuclear recruitment. There was, however, an increase in mitochondrial volume within myofibers in treated fish. GC/MS metabolomic analysis revealed shifts in amino acid composition of the musculature, putatively reflecting increases in connective tissue and decreases in protein synthesis that are associated with β-GPA treatment. These results suggest that β-GPA modestly affects fish muscle in a manner similar to that observed in mammals, and that β-GPA may have application to aquaculture by providing a more practical means of generating some of the beneficial effects of exercise in fishes.

Running training experience attenuates disuse atrophy in fast-twitch skeletal muscles of rats

Responsiveness to physiological stimuli, such as exercise and muscular inactivation, differs in individuals. However, the mechanisms responsible for these individual differences remain poorly understood. We tested whether a prior experience of exercise training affects the responses of skeletal muscles to unloading. Young rats were assigned to perform daily running training with a treadmill for 8 wk. After an additional 8 wk of normal habitation, the rats were hindlimb unloaded by tail suspension for 1 wk. Fast-twitch plantaris, gastrocnemius, and tibialis anterior muscles did not atrophy after unloading in rats with training experience, although soleus muscle lost weight similar to sedentary rats. We also analyzed the transcriptome in plantaris muscle with RNA sequencing followed by hierarchical clustering analysis and found that a subset of genes that were generally upregulated in sedentary rats after unloading were less responsive in rats with training experience. The distribution of histone 3 was diminished at the loci of these genes during the training period. Although the deposition of histone 3 was restored after an additional period of normal habitation, the incorporation of H3.3 variant was promoted in rats with training experience. This remodeling of nucleosomes closely correlated to the conformational changes of chromatin and suppressed gene expression in response to unloading. These results suggest that exercise training stimulated the early turnover of histone components, which may alter the responsiveness of gene transcription to physiological stimuli.NEW & NOTEWORTHY The present study demonstrates that disuse atrophy was suppressed in fast-twitch skeletal muscles of rats with training experience in early life. We also found a subset of genes that were less responsive to unloading in the muscle of rats with training experience. It was further determined that exercise training caused an early turnover of nucleosome components, which may alter the responsiveness of genes to stimulus in later life.

Systematic review of the synergist muscle ablation model for compensatory hypertrophy

Summary Objective: The aim was to evaluate the effectiveness of the experimental synergists muscle ablation model to promote muscle hypertrophy, determine the period of greatest hypertrophy and its influence on muscle fiber types and determine differences in bilateral and unilateral removal to reduce the number of animals used in this model. Method: Following the application of the eligibility criteria for the mechanical overload of the plantar muscle in rats, nineteen papers were included in the review. Results: The results reveal a greatest hypertrophy occurring between days 12 and 15, and based on the findings, synergist muscle ablation is an efficient model for achieving rapid hypertrophy and the contralateral limb can be used as there was no difference between unilateral and bilateral surgery, which reduces the number of animals used in this model. Conclusion: This model differs from other overload models (exercise and training) regarding the characteristics involved in the hypertrophy process (acute) and result in a chronic muscle adaptation with selective regulation and modification of fast-twitch fibers in skeletal muscle. This is an efficient and rapid model for compensatory hypertrophy.

Prenatal myonuclei play a crucial role in skeletal muscle hypertrophy in rodents

Multinucleated muscle fibers are formed by the fusion of myogenic progenitor cells during embryonic and fetal myogenesis. However, the role of prenatally incorporated myonuclei in the skeletal muscle fibers of adult animals is poorly understood. We demonstrated, using muscle-specific reporter mice, that the prenatal myonuclei remained in the adult soleus muscle, although cardiotoxin injection caused the loss of prenatal myonuclei. Overloading by the tendon transection of synergists failed to induce compensatory hypertrophy in regenerated soleus muscle fibers of adult rats, whereas unloading by tail suspension normally induced the fiber atrophy. Loss of hypertrophying function correlated with the lowered histone acetylation at the transcription start site of Igf1r gene, which was one of the genes that did not respond to the overloading. These parameters were improved by the transplantation of cells harvested from the juvenile soleus muscles of neonatal rats in association with enhanced histone acetylation of Igf1r gene. These results indicated that the presence of prenatal myonuclei was closely related to the status of histone acetylation, which could regulate the responsiveness of muscle fibers to physiological stimuli.

Nuclear bodies reorganize during myogenesis in vitro and are differentially disrupted by expression of FSHD-associated DUX4

Background

Nuclear bodies, such as nucleoli, PML bodies, and SC35 speckles, are dynamic sub-nuclear structures that regulate multiple genetic and epigenetic processes. Additional regulation is provided by RNA/DNA handling proteins, notably TDP-43 and FUS, which have been linked to ALS pathology. Previous work showed that mouse cell line myotubes have fewer but larger nucleoli than myoblasts, and we had found that nuclear aggregation of TDP-43 in human myotubes was induced by expression of DUX4-FL, a transcription factor that is aberrantly expressed and causes pathology in facioscapulohumeral dystrophy (FSHD). However, questions remained about nuclear bodies in human myogenesis and in muscle disease.

Methods

We examined nucleoli, PML bodies, SC35 speckles, TDP-43, and FUS in myoblasts and myotubes derived from healthy donors and from patients with FSHD, laminin-alpha-2-deficiency (MDC1A), and alpha-sarcoglycan-deficiency (LGMD2D). We further examined how these nuclear bodies and proteins were affected by DUX4-FL expression.

Results

We found that nucleoli, PML bodies, and SC35 speckles reorganized during differentiation in vitro, with all three becoming less abundant in myotube vs. myoblast nuclei. In addition, though PML bodies did not change in size, both nucleoli and SC35 speckles were larger in myotube than myoblast nuclei. Similar patterns of nuclear body reorganization occurred in healthy control, MDC1A, and LGMD2D cultures, as well as in the large fraction of nuclei that did not show DUX4-FL expression in FSHD cultures. In contrast, nuclei that expressed endogenous or exogenous DUX4-FL, though retaining normal nucleoli, showed disrupted morphology of some PML bodies and most SC35 speckles and also co-aggregation of FUS with TDP-43.

Conclusions

Nucleoli, PML bodies, and SC35 speckles reorganize during human myotube formation in vitro. These nuclear body reorganizations are likely needed to carry out the distinct gene transcription and splicing patterns that are induced upon myotube formation. DUX4-FL-induced disruption of some PML bodies and most SC35 speckles, along with co-aggregation of TDP-43 and FUS, could contribute to pathogenesis in FSHD, perhaps by locally interfering with genetic and epigenetic regulation of gene expression in the small subset of nuclei that express high levels of DUX4-FL at any one time.

Differences in histone modifications between slow- and fast-twitch muscle of adult rats and following overload, denervation, or valproic acid administration

Numerous studies have reported alterations in skeletal muscle properties and phenotypes in response to various stimuli such as exercise, unloading, and gene mutation. However, a shift in muscle fiber phenotype from fast twitch to slow twitch is not completely induced by stimuli. This limitation is hypothesized to result from the epigenetic differences between muscle types. The main purpose of the present study was to identify the differences in histone modification for the plantaris (fast) and soleus (slow) muscles of adult rats. Genome-wide analysis by chromatin immunoprecipitation followed by DNA sequencing revealed that trimethylation at lysine 4 and acetylation of histone 3, which occurs at transcriptionally active gene loci, was less prevalent in the genes specific to the slow-twitch soleus muscle. Conversely, gene loci specific to the fast-twitch plantaris muscle were associated with the aforementioned histone modifications. We also found that upregulation of slow genes in the plantaris muscle, which are related to enhanced muscular activity, is not associated with activating histone modifications. Furthermore, silencing of muscle activity by denervation caused the displacement of acetylated histone and RNA polymerase II (Pol II) in 5' ends of genes in plantaris, but minor effects were observed in soleus. Increased recruitment of Pol II induced by forced acetylation of histone was also suppressed in valproic acid-treated soleus. Our present data indicate that the slow-twitch soleus muscle has a unique set of histone modifications, which may relate to the preservation of the genetic backbone against physiological stimuli.

Responses of skeletal muscles to gravitational unloading and/or reloading

Adaptation of morphological, metabolic, and contractile properties of skeletal muscles to inhibition of antigravity activities by exposure to a microgravity environment or by simulation models, such as chronic bedrest in humans or hindlimb suspension in rodents, has been well reported. Such physiological adaptations are generally detrimental in daily life on earth. Since the development of suitable countermeasure(s) is essential to prevent or inhibit these adaptations, effects of neural, mechanical, and metabolic factors on these properties in both humans and animals were reviewed. Special attention was paid to the roles of the motoneurons (both efferent and afferent neurograms) and electromyogram activities as the neural factors, force development, and/or length of sarcomeres as the mechanical factors and mitochondrial bioenergetics as the metabolic factors.

Macrophage deficiency in osteopetrotic (op/op) mice inhibits activation of satellite cells and prevents hypertrophy in single soleus fibers

Effects of macrophage on the responses of soleus fiber size to hind limb unloading and reloading were studied in osteopetrotic homozygous (op/op) mice with inactivated mutation of macrophage colony-stimulating factor (M-CSF) gene and in wild-type (+/+) and heterozygous (+/op) mice. The basal levels of mitotically active and quiescent satellite cell (-46 and -39% vs. +/+, and -40 and -30% vs. +/op) and myonuclear number (-29% vs. +/+ and -28% vs. +/op) in fibers of op/op mice were significantly less than controls. Fiber length and sarcomere number in op/op were also less than +/+ (-22%) and +/op (-21%) mice. Similar trend was noted in fiber cross-sectional area (CSA, -15% vs. +/+, P = 0.06, and -14% vs. +/op, P = 0.07). The sizes of myonuclear domain, cytoplasmic volume per myonucleus, were identical in all types of mice. The CSA, length, and the whole number of sarcomeres, myonuclei, and mitotically active and quiescent satellite cells, as well as myonuclear domain, in single muscle fibers were decreased after 10 days of unloading in all types of mice, although all of these parameters in +/+ and +/op mice were increased toward the control values after 10 days of reloading. However, none of these levels in op/op mice were recovered. Data suggest that M-CSF and/or macrophages are important to activate satellite cells, which cause increase of myonuclear number during fiber hypertrophy. However, it is unclear why their responses to general growth and reloading after unloading are different.

Gene expression patterns, and protein metabolic and histological analyses for muscle development in Peking duck

In this study, we aimed to use duck breast muscle and leg muscle, the 2 main productive muscle organs, as a model to elucidate the molecular mechanism controlling how the 2 muscles have different deposition capabilities, and to analyze the mechanisms facilitating duck muscle development posthatching. Peking duck breast muscle and leg muscle were collected 3, 7, and 16 wk posthatching. The morphology of the myofibers was observed by paraffin sectioning the muscles. The expression of genes involved in protein metabolism [mammalian target of rapamycin (mTOR), RPS6-p70-protein kinase (S6K), forkhead box O1 (FoxO1), muscle RING finger 1 (MuRF1), and atrogin-1 (MAFbx)] was detected using real-time quantitative PCR and Western blot assays, and the results indicated that breast muscle had a stronger capacity for both protein synthesis and protein degradation compared with leg muscle. Satellite cell frequency declined during muscle development in both tissues, and the expression of Pax3/7, satellite cell marker genes, was not significantly different between breast muscle and leg muscle. No notable apoptosis was observed in either tissue. The results of this study suggest that protein metabolism signaling is the main reason promoting duck skeletal muscle mass gain.

p38 mitogen-activated protein kinase and mitogen-activated protein kinase-activated protein kinase 2 (MK2) signaling in atrophic and hypertrophic denervated mouse skeletal muscle

Background

p38 mitogen-activated protein kinase has been implicated in both skeletal muscle atrophy and hypertrophy. T317 phosphorylation of the p38 substrate mitogen-activated protein kinase-activated protein kinase 2 (MK2) correlates with muscle weight in atrophic and hypertrophic denervated muscle and may influence the nuclear and cytoplasmic distribution of p38 and/or MK2. The present study investigates expression and phosphorylation of p38, MK2 and related proteins in cytosolic and nuclear fractions from atrophic and hypertrophic 6-days denervated skeletal muscles compared to innervated controls.

Methods

Expression and phosphorylation of p38, MK2, Hsp25 (heat shock protein25_rodent/27_human, Hsp25/27) and Hsp70 protein expression were studied semi-quantitatively using Western blots with separated nuclear and cytosolic fractions from innervated and denervated hypertrophic hemidiaphragm and atrophic anterior tibial muscles. Unfractionated innervated and denervated atrophic pooled gastrocnemius and soleus muscles were also studied.

Results

No support was obtained for a differential nuclear/cytosolic localization of p38 or MK2 in denervated hypertrophic and atrophic muscle. The differential effect of denervation on T317 phosphorylation of MK2 in denervated hypertrophic and atrophic muscle was not reflected in p38 phosphorylation nor in the phosphorylation of the MK2 substrate Hsp25. Hsp25 phosphorylation increased 3-30-fold in all denervated muscles studied. The expression of Hsp70 increased 3-5-fold only in denervated hypertrophic muscles.

Conclusions

The study confirms a differential response of MK2 T317 phosphorylation in denervated hypertrophic and atrophic muscles and suggests that Hsp70 may be important for this. Increased Hsp25 phosphorylation in all denervated muscles studied indicates a role for factors other than MK2 in the regulation of this phosphorylation.

Effects of gravitational loading levels on protein expression related to metabolic and/or morphologic properties of mouse neck muscles

The effects of 3 months of spaceflight (SF), hindlimb suspension, or exposure to 2G on the characteristics of neck muscle in mice were studied. Three 8‐week‐old male C57BL/10J wild‐type mice were exposed to microgravity on the International Space Station in mouse drawer system (MDS) project, although only one mouse returned to the Earth alive. Housing of mice in a small MDS cage (11.6 × 9.8‐cm and 8.4‐cm height) and/or in a regular vivarium cage was also performed as the ground controls. Furthermore, ground‐based hindlimb suspension and 2G exposure by using animal centrifuge (n = 5 each group) were performed. SF‐related shift of fiber phenotype from type I to II and atrophy of type I fibers were noted. Shift of fiber phenotype was related to downregulation of mitochondrial proteins and upregulation of glycolytic proteins, suggesting a shift from oxidative to glycolytic metabolism. The responses of proteins related to calcium handling, myofibrillar structure, and heat stress were also closely related to the shift of muscular properties toward fast‐twitch type. Surprisingly, responses of proteins to 2G exposure and hindlimb suspension were similar to SF, although the shift of fiber types and atrophy were not statistically significant. These phenomena may be related to the behavior of mice that the relaxed posture without lifting their head up was maintained after about 2 weeks. It was suggested that inhibition of normal muscular activities associated with gravitational unloading causes significant changes in the protein expression related to metabolic and/or morphological properties in mouse neck muscle.

Inhibition of gravitational loading in space and on the Earth for 3 months caused similar responses of protein expression in mouse neck muscle. Downregulation of mitochondrial proteins and upregulation of glycolytic proteins were induced, suggesting a shift from oxidative to glycolytic metabolism. Furthermore, the responses of proteins, involved in calcium handling, myofibrillar structure, and heat stress, related to the shift of muscular properties toward fast‐twitch type were also noted. It was suggested that inhibition of normal muscular activities associated with gravitational unloading caused significant changes in the protein expression related to metabolic and/or morphological properties in mouse neck muscle.

Role of NO-synthase in regulation of protein metabolism of stretched rat m. soleus muscle during functional unloading

Gravitational unloading causes atrophy of muscle fibers and can lead to destruction of cytoskeletal and contractile proteins. Along with the atrophic changes, unloaded muscle frequently demonstrates significant shifts in the ratio of muscle fibers expressing fast and slow myosin heavy chain isoforms. Stretching of the m. soleus during hindlimb suspension prevents its atrophy. We supposed that neuronal NO-synthase (NOS) (which is attached to membrane dystrophin-sarcoglycan complex) can contribute to maintenance of protein metabolism in the muscle and prevent its atrophy when m. soleus is stretched. To test this hypothesis, we used Wistar rats (56 animals) in experiments with hindlimb suspension during 14 days. The group of hindlimb suspended rats with stretched m. soleus was injected with L-NAME to block NOS activity. We found that m. soleus mass and its protein content in hindlimb-suspended rats with stretched m. soleus were preserved due to prevention of protein degradation. NOS is involved in maintenance of expression of some muscle proteins. Proliferation of satellite cells in stretched m. soleus may be due to nNOS activity, but maintenance of muscle mass upon stretching is regulated not by NOS alone.

Akt (protein kinase B) isoform phosphorylation and signaling downstream of mTOR (mammalian target of rapamycin) in denervated atrophic and hypertrophic mouse skeletal muscle

BACKGROUND

The present study examines the hypothesis that Akt (protein kinase B)/mTOR (mammalian target of rapamycin) signaling is increased in hypertrophic and decreased in atrophic denervated muscle. Protein expression and phosphorylation of Akt1, Akt2, glycogen synthase kinase-3beta (GSK-3beta), eukaryotic initiation factor 4E binding protein 1 (4EBP1), 70 kD ribosomal protein S6 kinase (p70S6K1) and ribosomal protein S6 (rpS6) were examined in six-days denervated mouse anterior tibial (atrophic) and hemidiaphragm (hypertrophic) muscles.

RESULTS

In denervated hypertrophic muscle expression of total Akt1, Akt2, GSK-3beta, p70S6K1 and rpS6 proteins increased 2-10 fold whereas total 4EBP1 protein remained unaltered. In denervated atrophic muscle Akt1 and Akt2 total protein increased 2-16 fold. A small increase in expression of total rpS6 protein was also observed with no apparent changes in levels of total GSK-3beta, 4EBP1 or p70S6K1 proteins. The level of phosphorylated proteins increased 3-13 fold for all the proteins in hypertrophic denervated muscle. No significant changes in phosphorylated Akt1 or GSK-3beta were detected in atrophic denervated muscle. The phosphorylation levels of Akt2, 4EBP1, p70S6K1 and rpS6 were increased 2-18 fold in atrophic denervated muscle.

CONCLUSIONS

The results are consistent with increased Akt/mTOR signaling in hypertrophic skeletal muscle. Decreased levels of phosphorylated Akt (S473/S474) were not observed in denervated atrophic muscle and results downstream of mTOR indicate increased protein synthesis in denervated atrophic anterior tibial muscle as well as in denervated hypertrophic hemidiaphragm muscle. Increased protein degradation, rather than decreased protein synthesis, is likely to be responsible for the loss of muscle mass in denervated atrophic muscles.

Effects of mechanical over-loading on the properties of soleus muscle fibers, with or without damage, in wild type and mdx mice

Effects of mechanical over-loading on the characteristics of regenerating or normal soleus muscle fibers were studied in dystrophin-deficient (mdx) and wild type (WT) mice. Damage was also induced in WT mice by injection of cardiotoxin (CTX) into soleus muscle. Over-loading was applied for 14 days to the left soleus muscle in mdx and intact and CTX-injected WT mouse muscles by ablation of the distal tendons of plantaris and gastrocnemius muscles. All of the myonuclei in normal muscle of WT mice were distributed at the peripheral region. But, central myonuclei were noted in all fibers of WT mice regenerating from CTX-injection-related injury. Further, many fibers of mdx mice possessed central myonuclei and the distribution of such fibers was increased in response to over-loading, suggesting a shift of myonuclei from peripheral to central region. Approximately 1.4% branched fibers were seen in the intact muscle of mdx mice, although these fibers were not detected in WT mice. The percentage of these fibers in mdx, not in WT, mice was increased by over-loading (∼51.2%). The fiber CSA in normal WT mice was increased by over-loading (p<0.05), but not in mdx and CTX-injected WT mice. It was suggested that compensatory hypertrophy is induced in normal muscle fibers of WT mice following functional over-loading. But, it was also indicated that muscle fibers in mdx mice are susceptible to mechanical over-loading and fiber splitting and shift of myonuclei from peripheral to central region are induced.

Adaptation of mouse skeletal muscle to long-term microgravity in the MDS mission

The effect of microgravity on skeletal muscles has so far been examined in rat and mice only after short-term (5–20 day) spaceflights. The mice drawer system (MDS) program, sponsored by Italian Space Agency, for the first time aimed to investigate the consequences of long-term (91 days) exposure to microgravity in mice within the International Space Station. Muscle atrophy was present indistinctly in all fiber types of the slow-twitch soleus muscle, but was only slightly greater than that observed after 20 days of spaceflight. Myosin heavy chain analysis indicated a concomitant slow-to-fast transition of soleus. In addition, spaceflight induced translocation of sarcolemmal nitric oxide synthase-1 (NOS1) into the cytosol in soleus but not in the fast-twitch extensor digitorum longus (EDL) muscle. Most of the sarcolemmal ion channel subunits were up-regulated, more in soleus than EDL, whereas Ca²⁺-activated K⁺ channels were down-regulated, consistent with the phenotype transition. Gene expression of the atrophy-related ubiquitin-ligases was up-regulated in both spaceflown soleus and EDL muscles, whereas autophagy genes were in the control range. Muscle-specific IGF-1 and interleukin-6 were down-regulated in soleus but up-regulated in EDL. Also, various stress-related genes were up-regulated in spaceflown EDL, not in soleus. Altogether, these results suggest that EDL muscle may resist to microgravity-induced atrophy by activating compensatory and protective pathways. Our study shows the extended sensitivity of antigravity soleus muscle after prolonged exposition to microgravity, suggests possible mechanisms accounting for the resistance of EDL, and individuates some molecular targets for the development of countermeasures.

Effects of hindlimb unloading on neurogenesis in the hippocampus of newly weaned rats

Effects of hindlimb suspension (HS) and ambulation recovery on hippocampal neurogenesis of newly weaned rats were studied by using immunohistochemical techniques. The number of proliferating cell nuclear antigen-positive (PCNA(+)) cells in the subgranular zone (SGZ) markedly decreased during normal growth. However, neither HS nor subsequent recovery caused additional changes in the number of PCNA(+) cells. The number of doublecortin-positive (DCX(+)) neurons decreased gradually during normal growth. HS resulted in a further decrease in these neurons. However, DCX(+) cell numbers became identical to the levels in age-matched controls after 14 days of recovery. PCNA and DCX-double positive cells in the SGZ were also observed, and their cell numbers were not affected by HS and 14-day ambulation. Thus, HS suppressed the generation of DCX(+) neurons without affecting PCNA(+) cells in the SGZ of weaned rats. Taken together, hippocampal neurogenesis in weaned rats was not severely affected by HS while it decreased significantly as they had grown.

HSP25 can modulate myofibrillar desmin cytoskeleton following the phosphorylation at Ser15 in rat soleus muscle

The main purpose of the present study was to investigate the role(s) of 25-kDa heat shock protein (HSP25) in the regulation and integration of myofibrillar Z-disc structure during down- or upregulation of the size in rat soleus muscle fibers. Hindlimb unloading by tail suspension was performed in adult rats for 7 days, and reloading was allowed for 5 days after the termination of suspension. Interaction of HSP25 and Z-disc proteins, phosphorylation status, distribution, and complex formation of HSP25 were investigated. Non- and single-phosphorylated HSP25s were generally expressed in the cytoplasmic fraction of normal muscle. The level of total HSP25, as well as the phosphorylation ratio, did not change significantly in response to atrophy. Increased expressions of HSP25, phosphorylated at serine 15 (p-Ser15) and dual-phosphorylated form, were noted, when atrophied muscles were reloaded. Myofibrillar HSP25 was also noted in reloaded muscle. Histochemical analysis further indicated the localization of p-Ser15 in the regions with disorganization of Z-disc structure in reloaded muscle fibers. HSP25 formed a large molecular complex in the cytoplasmic fraction of normal muscle, whereas dissociation of free HSP25 with Ser15 phosphorylation was noted in reloaded muscle. The interaction of p-Ser15 with desmin and actinin was detected in Z-discs by proximity ligation assay. Strong interaction between p-Ser15 and desmin, but not actinin, was noted in the disorganized areas. These results indicated that HSP25 contributed to the desmin cytoskeletal organization following the phosphorylation at Ser15 during reloading and regrowing of soleus muscle.

Strength training elevates HSP27, HSP70 and αB-crystallin levels in musculi vastus lateralis and trapezius

A single bout of high-force exercise has been shown to increase the muscle levels of heat shock proteins (HSPs). Here, changes in the levels of HSPs after 2 and 11 weeks of strength training with either one or three sets per exercise were examined. Fifteen young men (27 ± 6 years, 182 ± 8 cm and 82 ± 13 kg) were randomized to train either one set in lower-body exercises and three sets in upper-body exercises (1L-3UB), or three sets in lower-body exercises and one set in upper-body exercises (3L-1UB). Biopsies from vastus lateralis and trapezius were obtained before, during (2 weeks) and after 11 weeks of strength training (3 bouts per week). The biopsies were analysed for HSP27 (cytosolic and cytoskeletal fractions) and HSP70 and αB-crystallin (cytosolic fraction). No evidence for an effect of training volume (1 vs. 3 sets) on the HSP response was found. For all subjects combined, HSP27 [186 ± 69% (mean ± SD)], HSP70 (146 ± 51%) and αB-crystallin (184 ± 82%) increased in the cytosolic fraction of vastus lateralis after 11 weeks of training. In the trapezius, the only observed increase was for HSP27 in the cytosolic fraction after 2 weeks of training (149 ± 59%). However, the trapezius contained somewhat higher levels of HSP70 and αB-crystallin than vastus lateralis at baseline. The HSP27 levels in the cytoskeletal compartment did not increase significantly in either muscle. In conclusion, strength training resulted-independent of training volume-in elevated levels of HSP27, HSP70 and αB-crystallin in the cytosolic compartment of the vastus lateralis. In the trapezius, only the cytosolic HSP27 levels were increased with training.

Region-specific responses of adductor longus muscle to gravitational load-dependent activity in Wistar Hannover rats

Response of adductor longus (AL) muscle to gravitational unloading and reloading was studied. Male Wistar Hannover rats (5-wk old) were hindlimb-unloaded for 16 days with or without 16-day ambulation recovery. The electromyogram (EMG) activity in AL decreased after acute unloading, but that in the rostral region was even elevated during continuous unloading. The EMG levels in the caudal region gradually increased up to 6th day, but decreased again. Approximately 97% of fibers in the caudal region were pure type I at the beginning of experiment. Mean percentage of type I fibers in the rostral region was 61% and that of type I+II and II fiber was 14 and 25%, respectively. The percent type I fibers decreased and de novo appearance of type I+II was noted after unloading. But the fiber phenotype in caudal, not rostral and middle, region was normalized after 16-day ambulation. Pronounced atrophy after unloading and re-growth following ambulation was noted in type I fibers of the caudal region. Sarcomere length in the caudal region was passively shortened during unloading, but that in the rostral region was unchanged or even stretched slightly. Growth-associated increase of myonuclear number seen in the caudal region of control rats was inhibited by unloading. Number of mitotic active satellite cells decreased after unloading only in the caudal region. It was indicated that the responses of fiber properties in AL to unloading and reloading were closely related to the region-specific neural and mechanical activities, being the caudal region more responsive.

Heat shock protein 90 regulates IκB kinase complex and NF-κB activation in angiotensin II-induced cardiac cell hypertrophy

Heat shock protein 90 (HSP90), one of the most abundant proteins in the cardiac cells is essential for cell survival. Previous studies have shown that angiotensin II induces cardiac cell hypertrophy. However, the role of HSP90 in the angiotensin II-induced cardiac hypertrophy is unclear. In this study, we showed that HSP90 regulated angiotensin II-induced hypertrophy via maintenance of the IκB kinase (IKK) complex stability in cardiac cells. An HSP90 inhibitor, geldanamycin (GA), significantly suppressed angiotensin II-induced [³H]leucine incorporation and atrial natriuretic factor expression in cardiac cells. GA also inhibited the NF-κB activation induced by angiotensin II. Importantly, treatment with GA caused a degradation of IKKα/β; on the other hand, a proteasome-specific inhibitor restored the level of IKKα/β. We also found that GA prevented HSP90-IKKs complex induced by angiotensin II in cardiac cells. The small interfering RNA (siRNA)-mediated knockdown of HSP90 expression significantly inhibited angiotensin II-induced cell hypertrophy and NF-κB activation. These results suggest that angiotensin II-induced cardiac hypertrophy requires HSP90 that regulates the stability and complex of IKK.

The time course of myonuclear accretion during hypertrophy in young adult and older rat plantaris muscle

To investigate whether accretion of myonuclei precedes or follows the increase in fibre cross-sectional area and whether this time course is affected by age, left plantaris muscle of 5- and 25-month-old male Wistar rats was overloaded by denervation of its synergists for 1, 2 or 4 weeks. Contralateral plantaris muscle served as control. Myonuclei were counted in haematoxylin-stained cross-sections. While hypertrophy (33% in young adult) became significant after 2 weeks overload (p<0.01), the myonuclear number was increased only at 4 weeks of overload (p<0.001). The time course and magnitude of hypertrophy were similar in young adult and older rats. Older muscles contained 26% more myonuclei per mm fibre length (p=0.001) and had a 10-fold larger proportion of central myonuclei (p<0.001) than young adult muscles. In conclusion, our data indicate that muscle fibre hypertrophy precedes the acquisition of new myonuclei and that the ability to develop hypertrophy is not attenuated or delayed in 25-month-old rat muscle.

Mitogen-activated protein kinase-activated protein kinase 2 (MK2) in skeletal muscle atrophy and hypertrophy

Skeletal muscle is a highly plastic tissue. Overall muscle growth (hypertrophy) or muscle wasting (atrophy) results from alterations in intracellular signaling pathways with important regulatory steps occurring in the nucleus as well as in the cytoplasm. Previous studies have identified components of the Akt/mTor pathway as well as the p38 MAPK pathway as important for skeletal muscle hypertrophy and/or atrophy. The present study tests the hypothesis that MK2, a substrate of p38 which following phosphorylation, can be exported from the nucleus in a complex with p38, may be important for skeletal muscle growth. The expression of MK2 was examined in denervated mouse hind-limb (atrophic) and hemidiaphragm (transiently hypertrophic) muscles. MK2 mRNA expression decreased after denervation in both atrophic (48% of innervated controls, P < 0.001) and hypertrophic muscle (34% of innervated controls, P < 0.01) but MK2 protein expression decreased only in atrophic muscle (32% of innervated controls, P < 0.01). The level of T205 phosphorylated MK2 increased after denervation in both atrophic (fourfold increase, P < 0.01) and hypertrophic muscles (almost sevenfold increase, P < 0.001) whereas the level of T317 phosphorylated MK2 (necessary for nuclear export) increased after denervation in hypertrophic muscle (nearly threefold increase, P < 0.001) but not in atrophic muscle. Logarithmically transformed relative changes in MK2 phosphorylated at T317 correlated well (r(2) = 0.7737) with relative changes in muscle weight. The results suggest a role for MK2 in the regulation of muscle mass, a role which, at least in part, may be related to determining the subcellular localization of p38 in muscle fibers.

PKA-induced F-actin rearrangement requires phosphorylation of Hsp27 by the MAPKAP kinase MK5

Mitogen-activated protein kinase (MAPK) pathways can play a role in F-actin dynamics. In particular, the p38 MAPK/MAPK-activated protein kinase 2 (MK2)/heat shock protein 27 (Hsp27) pathway is involved in F-actin alternations. Previously, we showed that MK5 is implicated in F-actin rearrangement induced by the cAMP/cAMP-dependent protein kinase pathway in PC12 cells, while others found Hsp27 to be a good in vitro MK5 substrate. Here we demonstrate that MK5 can specifically interact with Hsp27 in vivo and can induce phosphorylation at serine residues 78 and 82 in cells. siRNA-mediated depletion of Hsp27 protein levels, as well as overexpression of the non-phosphorylatable Hsp27-3A mutant prevented forskolin-induced F-actin reorganization. While ectopic expression of a constitutive active MK5 mutant was sufficient to induce F-actin rearrangement in PC12 cells, co-expression of Hsp27-3A could ablate this process. Our results imply that MK5 is involved in Hsp27-controlled F-actin dynamics in response to activation of the cAMP-dependent protein kinase pathway. These findings render the MK5/Hsp27 connection into a putative therapeutic target for conditions with aberrant Hsp27 phosphorylation such as metastasis, cardiovascular diseases, muscle atrophy, autoimmune skin disease and neuropathology.

The survival pathways phosphatidylinositol-3 kinase (PI3-K)/phosphoinositide-dependent protein kinase 1 (PDK1)/Akt modulate liver regeneration through hepatocyte size rather than proliferation

Liver regeneration comprises a series of complicated processes. The current study was designed to investigate the roles of phosphoinositide-dependent protein kinase 1 (PDK1)-associated pathways in liver regeneration after partial hepatectomy (PH) using liver-specific Pdk1-knockout (L-Pdk1KO) and Pdk1/STAT3 double KO (L-DKO) mice. There was no liver regeneration, and 70% PH was lethal in L-Pdk1KO mice. Liver regeneration was severely impaired equally in L-Pdk1KO and L-DKO mice, even after nonlethal 30% PH. There was no cell growth (measured as increase of cell size) after hepatectomy in L-Pdk1KO mice, although the post-PH mitotic response was the same as in controls. As expected, hepatectomy did not induce hepatic Akt-phosphorylation (Thr308) in L-Pdk1KO mice, and post-PH phosphorylation of Akt, mammalian target of rapamycin (mTOR), p70 ribosomal S6 kinase (p70(S6K)), and S6 were also reduced. To examine the specific role of PDK1-associated signals, a "pif-pocket" mutant of PDK1, which allows PDK1 only to phosphorylate Akt, was used. Liver regeneration was recovered in L-Pdk1KO mice with a "pif-pocket" mutant of PDK1. This re-activated Akt in L-Pdk1KO mice liver and induced post-PH cell growth, without affecting cell proliferation. Further deletion of STAT3 (L-DKO mice) did not further deteriorate liver regeneration, although this certainly reduced post-PH mitotic response. These findings indicate that PDK1/Akt contribute to liver regeneration by regulating cell size. Regarding phosphatidylinositol-3 kinase (PI3-K), immediate upstream signal of PDK1, activation of PI3-K induced cell proliferation via STAT3 activation in the liver of L-Pdk1KO mice but did not improve impaired liver regeneration. This confirmed the pivotal role of PDK1 in liver regeneration and cell growth.

CONCLUSION

PDK1/Akt-mediated responsive cell growth is essential for normal liver regeneration after PH, especially when cell proliferation is impaired.

Trypsin digest coupled with two-dimensional shotgun proteomics reveals the involvement of multiple signaling pathways in functional remodeling of late-gestation uteri in rats

Pregnant uteri become quiescent after functional remodeling but details are not fully known. Here we revealed uterine proteins of late-gestation rats by 2-D shotgun proteomic analysis and correlated protein expression with uterine functions. After duplication, 239 proteins were identified. About 190 proteins commonly found in duplicate analyses were subjected to functional annotation. The proteins associated with signal transduction fell into three known pathways. Western blotting and functional data indicated that: (i) a reduction of Na(+)/K(+)-ATPase-related proteins was associated with the decrease of contraction rate, (ii) a reduction of tyrosine hydroxylase and cyclic AMP-dependent protein kinase type II-alpha regulatory chain (PKARII alpha) was associated with an increase in the relaxation response to 8-bromo-cAMP, and (iii) in the presence of Ras, an increased expression of nucleolin was associated with the elevation of Bcl-xL, an antiapoptotic protein. In conclusion, 2-D shotgun proteomic analysis provides a global database of uterine proteins for hypothesis-driven studies. Our data suggest that in late-gestation uteri down-regulation of PKARII alpha and Na(+)/K(+)-ATPase may cause functional remodeling and lead to uterine quiescent. Up-regulation of antiapoptotic proteins (nucleolin and Bcl-xL) in the Ras-mediated pathway may maintain cell survival and counteract cell loss during remodeling.