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

Deficiency of heat shock transcription factor 1 suppresses heat stress-associated increase in slow soleus muscle mass of mice

AIM

Effects of heat shock transcription factor 1 (HSF1) deficiency on heat stress-associated increase in slow soleus muscle mass of mice were investigated.

METHODS

Both HSF1-null and wild-type mice were randomly assigned to control and heat-stressed groups. Mice in heat-stressed group were exposed to heat stress (41 °C for 60 min) in an incubator without anaesthesia.

RESULTS

Significant increase in wet and dry weights, and protein content of soleus muscle in wild-type mice was observed seven days after the application of the heat stress. However, heat stress had no impact on soleus muscle mass in HSF1-null mice. Neither type of mice exhibited much effect of heat stress on HSF mRNA expression (HSF1, HSF2 and HSF4). On the other hand, heat stress upregulated heat shock proteins (HSPs) at the mRNA (HSP72) and protein (HSP72 and HSP110) levels in wild-type mice, but not in HSF1-null mice. The population of Pax7-positive nuclei relative to total myonuclei of soleus muscle in wild-type mice was significantly increased by heat stress, but not in HSF1-null mice. Furthermore, the absence of HSF1 gene suppressed heat stress-associated phosphorylation of Akt and p70 S6 kinase (p-p70S6K) in soleus muscle.

CONCLUSION

Heat stress-associated increase in skeletal muscle mass may be induced by HSF1 and/or HSF1-mediated stress response that activates muscle satellite cells and Akt/p70S6K signalling pathway.

Intermittent stretching induces fibrosis in denervated rat muscle

INTRODUCTION

Stretching (St) has been used for treating denervated muscles. However, its effectiveness and safety claims require further study.

METHODS

Rats were divided into: (1) those with denervated (D) muscles, evaluated 7 or 15 days after sciatic nerve crush injury; (2) those with D muscles submitted to St during 7 or 15 days; and (3) those with normal muscles. Muscle fiber cross-sectional area, serial sarcomere number, sarcomere length, and connective tissue density were measured. MMP-2, MMP-9, TIMP-1, TGF-β1, and myostatin mRNAs were determined by real-time polymerase chain reaction. MMP-2 and MMP-9 activity was evaluated by zymography. Collagen I was localized using immunofluorescence.

RESULTS

St did not prevent muscle atrophy due to denervation, but it increased fibrosis and collagen I deposition at day 15. St also upregulated MMP-9 and TGF-β1 gene expressions at day 7, and myostatin at day 15.

CONCLUSIONS

Stretching denervated muscle does not prevent atrophy, but it increases fibrosis via temporal modulation of TGF-β1/myostatin and MMP-9 cascades.

Involvement of AMPK in regulating slow-twitch muscle atrophy during hindlimb unloading in mice

AMPK is considered to have a role in regulating skeletal muscle mass. However, there are no studies investigating the function of AMPK in modulating skeletal muscle mass during atrophic conditions. In the present study, we investigated the difference in unloading-associated muscle atrophy and molecular functions in response to 2-wk hindlimb suspension between transgenic mice overexpressing the dominant-negative mutant of AMPK (AMPK-DN) and their wild-type (WT) littermates. Male WT (n = 24) and AMPK-DN (n = 24) mice were randomly divided into two groups: an untreated preexperimental control group (n = 12 in each group) and an unloading (n = 12 in each group) group. The relative soleus muscle weight and fiber cross-sectional area to body weight were decreased by ∼30% in WT mice by hindlimb unloading and by ∼20% in AMPK-DN mice. There were no changes in puromycin-labeled protein or Akt/70-kDa ribosomal S6 kinase signaling, the indicators of protein synthesis. The expressions of ubiquitinated proteins and muscle RING finger 1 mRNA and protein, markers of the ubiquitin-proteasome system, were increased by hindlimb unloading in WT mice but not in AMPK-DN mice. The expressions of molecules related to the protein degradation system, phosphorylated forkhead box class O3a, inhibitor of κBα, microRNA (miR)-1, and miR-23a, were decreased only in WT mice in response to hindlimb unloading, and 72-kDa heat shock protein expression was higher in AMPK-DN mice than in WT mice. These results imply that AMPK partially regulates unloading-induced atrophy of slow-twitch muscle possibly through modulation of the protein degradation system, especially the ubiquitin-proteasome system.

Mechanical loading by fluid shear stress of myotube glycocalyx stimulates growth factor expression and nitric oxide production

Skeletal muscle fibers have the ability to increase their size in response to a mechanical overload. Finite element modeling data suggest that mechanically loaded muscles in vivo may experience not only tensile strain but also shear stress. However, whether shear stress affects biological pathways involved in muscle fiber size adaptation in response to mechanical loading is unknown. Therefore, our aim was twofold: (1) to determine whether shear stress affects growth factor expression and nitric oxide (NO) production by myotubes, and (2) to explore the mechanism by which shear stress may affect myotubes in vitro. C2C12 myotubes were subjected to a laminar pulsating fluid flow (PFF; mean shear stress 0.4, 0.7 or 1.4 Pa, 1 Hz) or subjected to uni-axial cyclic strain (CS; 15 % strain, 1 Hz) for 1 h. NO production during 1-h PFF or CS treatment was quantified using Griess reagent. The glycocalyx was degraded using hyaluronidase, and stretch-activated ion channels (SACs) were blocked using GdCl3. Gene expression was analyzed immediately after 1-h PFF (1.4 Pa, 1 Hz) and at 6 h post-PFF treatment. PFF increased IGF-I Ea, MGF, VEGF, IL-6, and COX-2 mRNA, but decreased myostatin mRNA expression. Shear stress enhanced NO production in a dose-dependent manner, while CS induced no quantifiable increase in NO production. Glycocalyx degradation and blocking of SACs ablated the shear stress-stimulated NO production. In conclusion, shear stress activates signaling pathways involved in muscle fiber size adaptation in myotubes, likely via membrane-bound mechanoreceptors. These results suggest that shear stress exerted on myofiber extracellular matrix plays an important role in mechanotransduction in muscle.

Multifidus Muscle Changes After Back Injury Are Characterized by Structural Remodeling of Muscle, Adipose and Connective Tissue, but Not Muscle Atrophy: Molecular and Morphological Evidence

STUDY DESIGN

Longitudinal case-controlled animal study.

OBJECTIVE

To investigate putative cellular mechanisms to explain structural changes in muscle and adipose and connective tissues of the back muscles after intervertebral disc (IVD) injury.

SUMMARY OF BACKGROUND DATA

Structural back muscle changes are ubiquitous with back pain/injury and considered relevant for outcome, but their exact nature, time course, and cellular mechanisms remain elusive. We used an animal model that produces phenotypic back muscle changes after IVD injury to study these issues at the cellular/molecular level.

METHODS

Multifidus muscle was harvested from both sides of the spine at L1-L2 and L3-L4 IVDs in 27 castrated male sheep at 3 (n = 10) or 6 (n = 17) months after a surgical anterolateral IVD injury at both levels. Ten control sheep underwent no surgery (3 mo, n = 4; 6 mo, n = 6). Tissue was harvested at L4 for histological analysis of cross-sectional area of muscle and adipose and connective tissue (whole muscle), plus immunohistochemistry to identify proportion and cross-sectional area of individual muscle fiber types in the deepest fascicle. Quantitative polymerase chain reaction measured gene expression of typical cytokines/signaling molecules at L2.

RESULTS

Contrary to predictions, there was no multifidus muscle atrophy (whole muscle or individual fiber). There was increased adipose and connective tissue (fibrotic proliferation) cross-sectional area and slow-to-fast muscle fiber transition at 6 but not 3 months. Within the multifidus muscle, increases in the expression of several cytokines (tumor necrosis factor α and interleukin-1β) and molecules that signal trophic/atrophic processes for the 3 tissue types (e.g., growth factor pathway [IGF-1, PI3k, Akt1, mTOR], potent tissue modifiers [calcineurin, PCG-1α, and myostatin]) were present.

CONCLUSION

This study provides cellular evidence that refutes the presence of multifidus muscle atrophy accompanying IVD degeneration at this intermediate time point. Instead, adipose/connective tissue increased in parallel with the expression of the genes that provide putative mechanisms for multifidus structural remodeling. This provides novel targets for pharmacological and physical interventions.

LEVEL OF EVIDENCE

N/A.

Influence of passive stretching on inhibition of disuse atrophy and hemodynamics of rat soleus muscle

The purpose of this study was to determine the influence of passive stretching on inhibition of disuse atrophy and hemodynamics among longitudinal regions of the rat soleus muscle. Disuse muscle atrophy was induced by hindlimb suspension for two weeks. Muscle blood flow was evaluated using thallium-201 ((201)Tl) which is a radiotracer that has been reported to be useful to assess blood perfusion in skeletal muscle. Thirty-nine male Wistar rats were divided randomly into 5 groups: control (C: n = 10), a group with hindlimb suspension (HS: n = 7), a group with hindlimb suspension and stretching (ST: n = 7), a group receiving only a single session of stretching after the hindlimb suspension period that was killed just after stretching (HSB: n = 7), and a group receiving only a single session of stretching hindlimb suspension and stretching period that was killed just after stretching (STB: n = 8). From the results of the cross-sectional area (CSA) and the capillary-to-fiber ratio (C/F), muscle atrophy and inhibition of atrophy were shown more in proximal than in distal regions of experimental groups. These results suggest that the alterations of the C/F and CSA were different among muscle regions in experimental groups. These differences may depend on the level of stretching. Moreover, alteration of blood flow resulting from alteration of the mechanical environment had little influence on muscle atrophy or inhibition of atrophy.

Neuromuscular adaptations associated with knee joint angle-specific force change

PURPOSE

Neuromuscular adaptations to joint angle-specific force increases after isometric training have not yet been fully elucidated. This study examined angle-specific neuromuscular adaptations in response to isometric knee extension training at short (SL, joint angle 38.1° ± 3.7°) versus long (LL, 87.5° ± 6.0°) muscle lengths.

METHODS

Sixteen men trained three times a week for 6 wk either at SL (n = 8) or LL (n = 8). Voluntary maximal isometric knee extensor (MVC) force, doublet twitch force, EMG amplitudes (EMG/Mmax), and voluntary activation during MVC force (VA%) were measured at eight knee joint angles (30°-100°) at weeks 0, 3, and 6. Muscle volume and cross-sectional area (CSA) were measured from magnetic resonance imaging scans, and fascicle length (Lf) was assessed using ultrasonography before and after training.

RESULTS

Clear joint angle specificity of force increase was seen in SL but not in LL. The 13.4% ± 9.7% (P = 0.01) force increase around the training angle in SL was related to changes in vastus lateralis and vastus medialis EMG/Mmax around the training angle (r = 0.84-0.88, P < 0.05), without changes in the doublet twitch force-angle relation or muscle size. In LL, muscle volume and CSA increased and the changes in CSA at specific muscle regions were correlated with changes in MVC force. A 5.4% ± 4.9% (P = 0.001) increase in Lf found in both groups was not associated with angle-specific force changes. There were no angle-specific changes in VA%.

CONCLUSION

The EMG/Mmax, although not VA%, results suggest that neural adaptations underpinned training-related changes at short quadriceps lengths, but hypertrophic changes predominated after training at long lengths. The findings of this study should contribute to the development of more effective and evidence-based rehabilitation and strength training protocols.

Loading-associated expression of TRIM72 and caveolin-3 in antigravitational soleus muscle in mice

Effects of mechanical loading on the expression level of tripartite motif-containing 72 (TRIM72) and caveolin-3 (Cav-3) in mouse soleus muscle were investigated. Mice were subjected to (1) continuous hindlimb suspension (HS) for 2 weeks followed by 1-week ambulation recovery or (2) functional overloading (FO) on the soleus by cutting the distal tendons of the plantaris and gastrocnemius muscles. Soleus muscle atrophy was induced by 2-week hindlimb suspension (HS). Reloading-associated regrowth of atrophied soleus muscle was observed by 1-week reloading following HS. HS also depressed the expression level of insulin receptor substrate-1 (IRS-1) mRNA, TRIM72, Cav-3, and phosphorylated Akt (p-Akt)/total Akt (t-Akt), but increased the phosphorylated level of p38 mitogen-activated protein kinase (p-p38MAPK) in soleus muscle. Thereafter, the expression level of MyoD mRNA, TRIM72 (mRNA, and protein), and Cav-3 was significantly increased and recovered to the basal level during 1-week reloading after HS. Although IRS-1 expression was also upregulated by reloading, the expression level was significantly lower than that before HS. Significant increase in p-Akt and phosphorylated p70 S6 kinase (p-p70S6K) was observed by 1-day reloading. On the other hand, 1-week functional overloading (FO) induced soleus muscle hypertrophy. In FO-associated hypertrophied soleus muscle, the expression level of IRS-1 mRNA, MyoD mRNA, TRIM72 mRNA, p-Akt, and p-p70S6K was increased, but the expression of Cav-3 and p-p38MAPK was decreased. FO had no effect on the protein expression level of TRIM72. These observations suggest that the loading-associated upregulation of TRIM72 protein in skeletal muscle may depress the regrowth of atrophied muscle via a partial suppression of IRS-1. In addition, downregulation of Cav-3 in skeletal muscle may depress overloading-induced muscle hypertrophy.

Muscle-specific GSK-3β ablation accelerates regeneration of disuse-atrophied skeletal muscle

Muscle wasting impairs physical performance, increases mortality and reduces medical intervention efficacy in chronic diseases and cancer. Developing proficient intervention strategies requires improved understanding of the molecular mechanisms governing muscle mass wasting and recovery. Involvement of muscle protein- and myonuclear turnover during recovery from muscle atrophy has received limited attention. The insulin-like growth factor (IGF)-I signaling pathway has been implicated in muscle mass regulation. As glycogen synthase kinase 3 (GSK-3) is inhibited by IGF-I signaling, we hypothesized that muscle-specific GSK-3β deletion facilitates the recovery of disuse-atrophied skeletal muscle. Wild-type mice and mice lacking muscle GSK-3β (MGSK-3β KO) were subjected to a hindlimb suspension model of reversible disuse-induced muscle atrophy and followed during recovery. Indices of muscle mass, protein synthesis and proteolysis, and post-natal myogenesis which contribute to myonuclear accretion, were monitored during the reloading of atrophied muscle. Early muscle mass recovery occurred more rapidly in MGSK-3β KO muscle. Reloading-associated changes in muscle protein turnover were not affected by GSK-3β ablation. However, coherent effects were observed in the extent and kinetics of satellite cell activation, proliferation and myogenic differentiation observed during reloading, suggestive of increased myonuclear accretion in regenerating skeletal muscle lacking GSK-3β. This study demonstrates that muscle mass recovery and post-natal myogenesis from disuse-atrophy are accelerated in the absence of GSK-3β.

Mass spectrometric characterization of a biotechnologically produced full-length mechano growth factor (MGF) relevant for doping controls

OBJECTIVE

Since Goldspink and colleagues identified the expression of the mRNA of an insulin-like growth factor 1 (IGF-1) isoform in response to mechanical stress in 1996, substantial research into the so-called mechano growth factor and its modus operandi followed until today. Promising preclinical results were obtained by using the synthetic, 24-amino acid residues spanning peptide translated from the exons 4-6 of IGF-1Ec (which was later referred to as the mechano growth factor (MGF) peptide), particularly with regard to increased muscle myoblast proliferation. Consequently, the MGF peptide represented a promising drug candidate for the treatment of neuromuscular disorders; however, its misuse potential in sport was also identified shortly thereafter, and the substance (or class of substances) has been considered prohibited according to the regulations of the World Anti-Doping Agency (WADA) since 2005. While various MGF peptide versions have been known to sports drug testing authorities, the occurrence of a 'full-length MGF' as offered via illicit channels to athletes or athletes' managers was reported in 2014, arguably being undetectable in doping controls.

METHODS

An aliquot of the product was obtained and the content characterized by state-of-the-art analytical approaches including gel electrophoretic and mass spectrometric (top-down and bottom-up) sequencing approaches. Upon full characterization, its implementation into modified routine doping controls using ultrafiltration, immunoaffinity-based isolation, and nanoliquid chromatography-high resolution/high accuracy mass spectrometry was established.

RESULTS

A protein with a monoisotopic molecular mass of 12264.9 Da and a sequence closely related to IGF-1Ec (lacking the signal- and propeptide moiety) was identified. The C-terminus was found to be modified by the elimination of the terminal lysine and a R109H substitution. With the knowledge of the compound's composition, existing doping control assays targeting peptide hormones such as IGF-1 and related substances were assessed as to their capability to detect the full-length MGF. The analyte was detectable at concentrations of 0.25 ng/mL using adapted routine test methods employing immunoaffinity purification followed by nanoscale liquid chromatography-high resolution/high accuracy (tandem) mass spectrometry.

CONCLUSIONS

A potentially performance enhancing 'full-length' MGF derivative was identified and successfully implemented into sports drug testing protocols. Future tests are indicated probing for optimized/dedicated detection methods and assessment of efficacy and elimination kinetics of the substance.

Effects of eccentric exercise in rehabilitation of phasic and tonic muscles after leg immobilization in rats

Eccentric exercise is an essential resource for skeletal muscle rehabilitation following muscle disuse however, abnormalities linked to the tissue recuperation require further research. Our aim was analyze the adaptation ability of rehabilitated muscular tissue in rats during different periods of eccentric training after 10 days of limb immobilization. Twenty-seven Wistar rats were divided into six groups: immobilized 10 days, immobilized and eccentric trained for 10 days, immobilized and eccentric trained for 21 days, and three age-matched control groups. After sacrifice, soleus and plantaris muscles were frozen, cut and stained for general histology using hematoxylin and eosin and Gomori trichrome methods and immunohistochemical methods for fiber typing (mATPase, NADH2-TR), for capillaries (CD31) and intermediate filaments (desmin, vimentin) and high resolution microscopy of resin embedded material. Immobilization resulted in more intense morphological alterations in soleus muscles such as formation of target fibers, nuclear centralization, a reduction in the number of type I fibers, diameter of type I, IIA, IIAD fibers, and capillaries. After 10 days of eccentric training, increases in the nuclear centralization and the number of lobulated fibers were observed. This period was insufficient to reestablish the capillary/fiber (C/F) ratio and distribution of fiber types as that observed in the control group. However, 21 days of rehabilitation allowed the reversal of all morphological and quantitative abnormalities. For the plantaris muscles, 10-days of training restored their basic characteristics. Despite the fact that immobilization affected soleus and plantaris muscles, 10 days of eccentric training was insufficient to restore the morphological characteristics of soleus muscles, which was not the case observed in plantaris muscle.

Impact of range of motion during ecologically valid resistance training protocols on muscle size, subcutaneous fat, and strength

The impact of using different resistance training (RT) kinematics, which therefore alters RT mechanics, and their subsequent effect on adaptations remain largely unreported. The aim of this study was to identify the differences to training at a longer (LR) compared with a shorter (SR) range of motion (ROM) and the time course of any changes during detraining. Recreationally active participants in LR (aged 19 ± 2.6 years; n = 8) and SR (aged 19 ± 3.4 years; n = 8) groups undertook 8 weeks of RT and 4 weeks of detraining. Muscle size, architecture, subcutaneous fat, and strength were measured at weeks 0, 8, 10, and 12 (repeated measures). A control group (aged 23 ± 2.4 years; n = 10) was also monitored during this period. Significant (p > 0.05) posttraining differences existed in strength (on average 4 ± 2 vs. 18 ± 2%), distal anatomical cross-sectional area (59 ± 15 vs. 16 ± 10%), fascicle length (23 ± 5 vs. 10 ± 2%), and subcutaneous fat (22 ± 8 vs. 5 ± 2%), with LR exhibiting greater adaptations than SR. Detraining resulted in significant (p > 0.05) deteriorations in all muscle parameters measured in both groups, with the SR group experiencing a more rapid relative loss of postexercise increases in strength than that experienced by the LR group (p > 0.05). Greater morphological and architectural RT adaptations in the LR (owing to higher mechanical stress) result in a more significant increase in strength compared with that of the SR. The practical implications for this body of work follow that LR should be observed in RT where increased muscle strength and size are the objective, because we demonstrate here that ROM should not be compromised for greater external loading.

Use it or lose it: multiscale skeletal muscle adaptation to mechanical stimuli

Skeletal muscle undergoes continuous turnover to adapt to changes in its mechanical environment. Overload increases muscle mass, whereas underload decreases muscle mass. These changes are correlated with, and enabled by, structural alterations across the molecular, subcellular, cellular, tissue, and organ scales. Despite extensive research on muscle adaptation at the individual scales, the interaction of the underlying mechanisms across the scales remains poorly understood. Here, we present a thorough review and a broad classification of multiscale muscle adaptation in response to a variety of mechanical stimuli. From this classification, we suggest that a mathematical model for skeletal muscle adaptation should include the four major stimuli, overstretch, understretch, overload, and underload, and the five key players in skeletal muscle adaptation, myosin heavy chain isoform, serial sarcomere number, parallel sarcomere number, pennation angle, and extracellular matrix composition. Including this information in multiscale computational models of muscle will shape our understanding of the interacting mechanisms of skeletal muscle adaptation across the scales. Ultimately, this will allow us to rationalize the design of exercise and rehabilitation programs, and improve the long-term success of interventional treatment in musculoskeletal disease.

Basic models modeling resistance training: an update for basic scientists interested in study skeletal muscle hypertrophy

Human muscle hypertrophy brought about by voluntary exercise in laboratorial conditions is the most common way to study resistance exercise training, especially because of its reliability, stimulus control and easy application to resistance training exercise sessions at fitness centers. However, because of the complexity of blood factors and organs involved, invasive data is difficult to obtain in human exercise training studies due to the integration of several organs, including adipose tissue, liver, brain and skeletal muscle. In contrast, studying skeletal muscle remodeling in animal models are easier to perform as the organs can be easily obtained after euthanasia; however, not all models of resistance training in animals displays a robust capacity to hypertrophy the desired muscle. Moreover, some models of resistance training rely on voluntary effort, which complicates the results observed when animal models are employed since voluntary capacity is something theoretically impossible to measure in rodents. With this information in mind, we will review the modalities used to simulate resistance training in animals in order to present to investigators the benefits and risks of different animal models capable to provoke skeletal muscle hypertrophy. Our second objective is to help investigators analyze and select the experimental resistance training model that best promotes the research question and desired endpoints.

Postnatal muscle modification by myogenic factors modulates neuropathology and survival in an ALS mouse model

MyoD and myogenin are myogenic transcription factors preferentially expressed in adult fast and slow muscles, respectively. Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder in which motor neuron loss is accompanied by muscle denervation and paralysis. Studies suggest that muscle phenotype may influence ALS disease progression. Here we demonstrate that myogenin gene transfer into muscle supports spinal cord motor neuron survival and muscle endplate innervation in the G93A SOD1 fALS mice. On the other hand, MyoD gene transfer decreases survival and enhances motor neuron degeneration and muscle denervation. Although an increase in motor neuron count is associated with increased succinic dehydrogenase staining in the muscle, muscle overexpression of PGC-1α does not improve survival or motor function. Our study suggests that postnatal muscle modification influences disease progression and demonstrates that the muscle expression of myogenic and metabolic regulators differentially impact neuropathology associated with disease progression in the G93A SOD1 fALS mouse model.

Effects of three protocols of hamstring muscle stretching and paravertebral lumbar

Introduction the muscle stretching is widely used to gain extensibility and flexibility, it is important to know the duration of these effects, after return to usual activity level. Thus, the aim of this study was to analyze the effect of three protocols of hamstring and paravertebral lumbar muscles stretching, and joint flexibility and muscle extensibility after six weeks. Methods participants were 40 volunteers, with limited hamstring extensibility, randomized into three groups: active stretching static (n = 14), proprioceptive neuromuscular facilitation (n = 14) and kinesiostretching (n = 12). The protocol was divided into 3 stages: the 1st control (six weeks), the 2nd application of stretch (six weeks) and the 3rd follow-up (eight weeks). The project was approved by the Ethics Committee on Human Research Unioeste, under protocol number 25536/2008. Four evaluations were conducted with board coupled to a system and goniometry and Well´s bench, distributed at the beginning and end of each step. Data were analyzed with repeated measures ANOVA, and one-way, with a significance level of 5%. Results there was no significant difference for the three groups in the control stage. There were significant differences in the three protocols in the stage of stretching. After follow-up stage, there was significant difference in the ratings to the board goniometry, and there was no difference in the Well’s Bench. Conclusion the three techniques promoted significant gain in extensibility and flexibility, extensibility was not maintained after the follow-up stage, and the flexibility of the posterior chain continued gains.

Muscular adaptations and insulin-like growth factor-1 responses to resistance training are stretch-mediated

INTRODUCTION

Modulation of muscle characteristics was attempted through altering muscle stretch during resistance training. We hypothesized that stretch would enhance muscle responses.

METHODS

Participants trained for 8 weeks, loading the quadriceps in a shortened (SL, 0-50° knee flexion; n=10) or lengthened (LL, 40-90°; n=11) position, followed by 4 weeks of detraining. Controls (CON; n=10) were untrained. Quadriceps strength, vastus lateralis architecture, anatomical cross-sectional area (aCSA), and serum insulin-like growth factor-1 (IGF-1) were measured at weeks 0, 8, 10, and 12.

RESULTS

Increases in fascicle length (29±4% vs. 14±4%), distal aCSA (53±12% vs. 18±8%), strength (26±6% vs. 7±3%), and IGF-1 (31±6% vs. 7±6%) were greater in LL compared with SL muscles (P<0.05). No changes occurred in CON. Detraining decrements in strength and aCSA were greater in SL than LL muscles (P<0.05).

CONCLUSIONS

Enhanced muscle in vivo (and somewhat IGF-1) adaptations to resistance training are concurrent with muscle stretch, which warrants its inclusion within training.

AICAR-induced activation of AMPK negatively regulates myotube hypertrophy through the HSP72-mediated pathway in C2C12 skeletal muscle cells

5'-AMP-activated protein kinase (AMPK) plays an important role as a negative regulator of skeletal muscle mass. However, the precise mechanism of AMPK-mediated regulation of muscle mass is not fully clarified. Heat shock proteins (HSPs), stress-induced molecular chaperones, are related with skeletal muscle adaptation, but the association between AMPK and HSPs in skeletal muscle hypertrophy is unknown. Thus, we investigated whether AMPK regulates hypertrophy by mediating HSPs in C2C12 cells. The treatment with AICAR, a potent stimulator of AMPK, decreased 72-kDa HSP (HSP72) expression, whereas there were no changes in the expressions of 25-kDa HSP, 70-kDa heat shock cognate, and heat shock transcription factor 1 in myotubes. Protein content and diameter were less in the AICAR-treated myotubes in those without treatment. AICAR-induced suppression of myotube hypertrophy and HSP72 expression was attenuated in the siRNA-mediated AMPKα knockdown myotubes. AICAR increased microRNA (miR)-1, a modulator of HSP72, and the increase of miR-1 was not induced in AMPKα knockdown condition. Furthermore, siRNA-mediated HSP72 knockdown blocked AICAR-induced inhibition of myotube hypertrophy. AICAR upregulated the gene expression of muscle Ring-finger 1, and this alteration was suppressed in either AMPKα or HSP72 knockdown myotubes. The phosphorylation of p70 S6 kinase Thr(389) was downregulated by AICAR, whereas this was attenuated in AMPKα, but not in HSP72, knockdown myotubes. These results suggest that AMPK inhibits hypertrophy through, in part, an HSP72-associated mechanism via miR-1 and protein degradation pathways in skeletal muscle cells.

Up-regulation of adiponectin expression in antigravitational soleus muscle in response to unloading followed by reloading, and functional overloading in mice

The purpose of this study was to investigate the expression level of adiponectin and its related molecules in hypertrophied and atrophied skeletal muscle in mice. The expression was also evaluated in C2C12 myoblasts and myotubes. Both mRNA and protein expression of adiponectin, mRNA expression of adiponectin receptor (AdipoR) 1 and AdipoR2, and protein expression of adaptor protein containing pleckstrin homology domain, phosphotyrosine binding domain, and leucine zipper motif 1 (APPL1) were observed in C2C12 myoblasts. The expression levels of these molecules in myotubes were higher than those in myoblasts. The expression of adiponectin-related molecules in soleus muscle was observed at mRNA (adiponectin, AdipoR1, AdipoR2) and protein (adiponectin, APPL1) levels. The protein expression levels of adiponectin and APPL1 were up-regulated by 3 weeks of functional overloading. Down-regulation of AdipoR1 mRNA, but not AdipoR2 mRNA, was observed in atrophied soleus muscle. The expression of adiponectin protein, AdipoR1 mRNA, and APPL1 protein was up-regulated during regrowth of unloading-associated atrophied soleus muscle. Mechanical loading, which could increase skeletal muscle mass, might be a useful stimulus for the up-regulations of adiponectin and its related molecules in skeletal muscle.

eIF4EBP3L acts as a gatekeeper of TORC1 in activity-dependent muscle growth by specifically regulating Mef2ca translational initiation

Muscle activity promotes muscle growth through the TOR-4EBP pathway by controlling the translation of specific mRNAs, including Mef2ca, a muscle transcription factor required for normal growth.

Muscle fiber size is activity-dependent and clinically important in ageing, bed-rest, and cachexia, where muscle weakening leads to disability, prolonged recovery times, and increased costs. Inactivity causes muscle wasting by triggering protein degradation and may simultaneously prevent protein synthesis. During development, muscle tissue grows by several mechanisms, including hypertrophy of existing fibers. As in other tissues, the TOR pathway plays a key role in promoting muscle protein synthesis by inhibition of eIF4EBPs (eukaryotic Initiation Factor 4E Binding Proteins), regulators of the translational initiation. Here, we tested the role of TOR-eIF4EBP in a novel zebrafish muscle inactivity model. Inactivity triggered up-regulation of eIF4EBP3L (a zebrafish homolog of eIF4EBP3) and diminished myosin and actin content, myofibrilogenesis, and fiber growth. The changes were accompanied by preferential reduction of the muscle transcription factor Mef2c, relative to Myod and Vinculin. Polysomal fractionation showed that Mef2c decrease was due to reduced translation of mef2ca mRNA. Loss of Mef2ca function reduced normal muscle growth and diminished the reduction in growth caused by inactivity. We identify eIF4EBP3L as a key regulator of Mef2c translation and protein level following inactivity; blocking eIF4EBP3L function increased Mef2ca translation. Such blockade also prevented the decline in mef2ca translation and level of Mef2c and slow myosin heavy chain proteins caused by inactivity. Conversely, overexpression of active eIF4EBP3L mimicked inactivity by decreasing the proportion of mef2ca mRNA in polysomes, the levels of Mef2c and slow myosin heavy chain, and myofibril content. Inhibiting the TOR pathway without the increase in eIF4EBP3L had a lesser effect on myofibrilogenesis and muscle size. These findings identify eIF4EBP3L as a key TOR-dependent regulator of muscle fiber size in response to activity. We suggest that by selectively inhibiting translational initiation of mef2ca and other mRNAs, eIF4EBP3L reprograms the translational profile of muscle, enabling it to adjust to new environmental conditions.

Most genes are transcribed into mRNA and then translated into proteins that function in various cellular processes. Initiation of mRNA translation is thus a fundamental control point in gene expression. Working in a zebrafish model, we have found that muscle activity (or inactivity) can differentially regulate the translation of specific mRNAs and thereby control the growth of skeletal muscle. Emerging evidence suggests that control of translational initiation of particular mRNAs by an intracellular signaling pathway acting through TORC1 is a major regulator of cell growth and function. We show here that muscle activity both activates the TORC1 pathway and suppresses the expression of a downstream TORC1 target—the translational inhibitor eIF4EBP3L. This removes a brake on translation of certain mRNAs. Conversely, we show that muscle inactivity can up-regulate this translational inhibitor, thereby causing reduced translation of these mRNAs. One of the mRNAs targeted in this manner by eIF4EBP3L is Mef2ca, which encodes a transcription factor that promotes assembly of muscle contractile apparatus. Our work thus reveals a mechanism by which muscle growth can be differentially influenced depending on the context of muscle activity (or lack thereof). If this pathway operates in people, it may help explain how exercise regulates muscle growth and performance.

Mechano-regulation of alternative splicing

Alternative splicing contributes to the complexity of proteome by producing multiple mRNAs from a single gene. Affymetrix exon arrays and experiments in vivo or in vitro demonstrated that alternative splicing was regulated by mechanical stress. Expression of mechano-growth factor (MGF) which is the splicing isoform of insulin-like growth factor 1(IGF-1) and vascular endothelial growth factor (VEGF) splicing variants such as VEGF₁₂₁, VEGF_165, VEGF₂₀₆, VEGF₁₈₉, VEGF₁₆₅ and VEGF₁₄₅ are regulated by mechanical stress. However, the mechanism of this process is not yet clear. Increasing evidences showed that the possible mechanism is related to Ca²⁺ signal pathway and phosphorylation signal pathway. This review proposes possible mechanisms of mechanical splicing regulation. This will contribute to the biomechanical study of alternative splicing.

Quantitative changes in focal adhesion kinase and its inhibitor, FRNK, drive load-dependent expression of costamere components

Costameres are mechanosensory sites of focal adhesion in the sarcolemma that reinforce the muscle-fiber composite and provide an anchor for myofibrillogenesis. We hypothesized that elevated content of the integrin-associated regulator of costamere turnover in culture, focal adhesion kinase (FAK), drives changes in costamere component content in antigravity muscle in a load-dependent way in correspondence with altered muscle weight. The content of FAK in soleus muscle being phosphorylated at autoregulatory tyrosine 397 (FAK-pY397) was increased after 20 s of stretch. FAK-pY397 content remained elevated after 24 h of stretch-overload due to upregulated FAK content. Overexpression of FAK in soleus muscle fibers by means of gene electrotransfer increased the β1-integrin (+56%) and meta-vinculin (+88%) content. α7-Integrin (P = 0.46) and γ-vinculin (P = 0.18) content was not altered after FAK overexpression. Co-overexpression of the FAK inhibitor FAK-related nonkinase (FRNK) reduced FAK-pY397 content by 33% and increased the percentage of fast-type fibers that arose in connection with hybrid fibers with gene transfer. Transplantation experiments confirmed the association of FRNK expression with slow-to-fast fiber transformation. Seven days of unloading blunted the elevation of FAK-pY397, β1-integrin, and meta-vinculin content with FAK overexpression, and this was reversed by 1 day of reloading. The results highlight that the expression of components for costameric attachment sites of myofibrils is under load- and fiber type-related control via FAK and its inhibitor FRNK.

Non-uniform muscle oxygenation despite uniform neuromuscular activity within the vastus lateralis during fatiguing heavy resistance exercise

Previous studies have reported for the vastus lateralis (VL) that the extent of muscle hypertrophy in response to resistance training is greater in the distal than in the middle region, despite uniform muscle fibre composition within VL along its length. In the present study, to investigate mechanism(s) for such non-uniform muscle hypertrophy, we simultaneously measured neuromuscular activity and muscle oxygenation state at the middle and distal regions of VL during fatiguing heavy resistance exercise. Twelve males performed unilateral knee extension exercise which consisted of 4 sets of 8 repetitions at intensity of 80% of the individual one repetition maximum. During the resistance exercise, neuromuscular activities and muscle oxygenation status at the middle and distal regions (50% and 70% of the thigh length, respectively) of VL were measured by using electromyography and near-infrared spectroscopy, respectively. Neuromuscular activities were similar between the distal and middle regions of VL, whereas muscle tissue oxygenation saturation was significantly lower at the distal than at the middle region of VL. These results suggest a possibility that the regional difference in muscle oxygenation but not in neuromuscular activity during fatiguing heavy resistance exercise is responsible for the regional difference in hypertrophy within a muscle.

Vascular-targeted therapies for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most common muscular dystrophy and an X-linked recessive, progressive muscle wasting disease caused by the absence of a functional dystrophin protein. Dystrophin has a structural role as a cytoskeletal stabilization protein and protects cells against contraction-induced damage. Dystrophin also serves a signaling role through mechanotransduction of forces and localization of neuronal nitric oxide synthase (nNOS), which produces nitric oxide (NO) to facilitate vasorelaxation. In DMD, the signaling defects produce inadequate tissue perfusion caused by functional ischemia due to a diminished ability to respond to shear stress induced endothelium-dependent dilation. Additionally, the structural defects seen in DMD render myocytes with an increased susceptibility to mechanical stress. The combination of both defects is necessary to generate myocyte damage, which induces successive rounds of myofiber degeneration and regeneration, loss of calcium homeostasis, chronic inflammatory response, fibrosis, and myonecrosis. In individuals with DMD, these processes inevitably cause loss of ambulation shortly after the first decade and an abbreviated life with death in the third or fourth decade due to cardio-respiratory anomalies. There is no known cure for DMD, and although the culpable gene has been identified for more than twenty years, research on treatments has produced few clinically relevant results. Several recent studies on novel DMD therapeutics are vascular targeted and focused on attenuating the inherent functional ischemia. One approach improves vasorelaxation capacity through pharmaceutical inhibition of either phosphodiesterase 5 (PDE5) or angiotensin-converting enzyme (ACE). Another approach increases the density of the underlying vascular network by inducing angiogenesis, and this has been accomplished through either direct delivery of vascular endothelial growth factor (VEGF) or by downregulating the VEGF decoy-receptor type 1 (VEGFR-1 or Flt-1). The pro-angiogenic approaches also seem to be pro-myogenic and could resolve the age-related decline in satellite cell (SC) quantity seen in mdx models through expansion of the SC juxtavascular niche. Here we review these four vascular targeted treatment strategies for DMD and discuss mechanisms, proof of concept, and the potential for clinical relevance associated with each therapy.

Treinamento aeróbico prévio à compressão nervosa: análise da morfometria muscular de ratos

INTRODUÇÃO: Ciatalgia origina-se da compressão do nervo isquiático e implica em dor, parestesia, diminuição da força muscular e hipotrofia. O exercício físico é reconhecido na prevenção e reabilitação de lesões, porém quando em sobrecargas pode aumentar o risco de lesões e consequente déficit funcional. OBJETIVO: Avaliar efeitos de treinamento aeróbico prévio a modelo experimental de ciatalgia em relação a parâmetros morfométricos dos músculos sóleos de ratos. MATERIAIS E MÉTODOS: 18 ratos divididos em três grupos: simulacro (mergulho, 30 segundos); exercício regular (natação, dez minutos diários); e treinamento aeróbico progressivo (natação em tempos progressivos de dez a 60 minutos diários). Ao final de seis semanas de exercício, os ratos foram submetidos ao modelo experimental da ciatalgia. No terceiro dia após a lesão, foram eutanasiados e tiveram seus músculos sóleos dissecados, pesados e preparados para análise histológica. Variáveis analisadas: peso muscular, área de secção transversa e diâmetro médio das fibras musculares. RESULTADOS: Observou-se diferença estatisticamente significativa para todos os grupos quando se comparou músculo controle e aquele submetido à lesão isquiática. A análise intergrupos não apresentou diferença estatisticamente significativa para nenhuma das variáveis analisadas. CONCLUSÃO: Tanto o exercício físico regular quanto o treinamento aeróbico não produziram efeitos preventivos ou agravantes às consequências musculares da inatividade funcional após ciatalgia.

Outcome of Grade I and II Hamstring Injuries in Intercollegiate Athletes: A Novel Rehabilitation Protocol

Background:

Hamstring muscle strains represent a common and disabling athletic injury with variable recurrence rates and prolonged recovery times.

Objectives:

To present the outcomes of a novel rehabilitation protocol for the treatment of proximal hamstring strains in an intercollegiate sporting population and to determine any significant differences in the rate of reinjury and time to return to sport based on patient and injury characteristics.

Study Design:

Retrospective case series.

Methods:

A retrospective review was performed of 48 consecutive hamstring strains in intercollegiate athletes. The rehabilitation protocol consisted of early mobilization, with flexible progression through supervised drills. Athletes were allowed to return to sport after return of symmetrical strength and range of motion with no pain during sprinting. Primary outcomes included time to return to sport and reinjury rates.

Results:

All patients returned to their sports, and 3 sustained repeat hamstring strains (6.2% reinjury rate) after a minimum follow-up of 6 months. The average number of days missed from sport was 11.9 (range, 5-23 days). There was no statistically significant difference for time to return to sport between first-time and recurrent injuries and between first- and second-degree injuries (P > 0.05).

Conclusions:

Grade I and II hamstring strains may be aggressively treated with a protocol of brief immobilization followed by early initiation of running and isokinetic exercises—with an average expected return to sport of approximately 2 weeks and with a relatively low reinjury rate regardless of injury grade (I or II), injury characteristics (including first-time and recurrent injuries), or athlete characteristics.

Atrophy/hypertrophy cell signaling in muscles of young athletes trained with vibrational-proprioceptive stimulation

OBJECTIVE

To compare the effects of isokinetic (ISO-K) and vibrational-proprioceptive (VIB) trainings on muscle mass and strength.

METHODS

In 29 ISO-K- or VIB-trained young athletes we evaluated: force, muscle fiber morphometry, and gene expression of muscle atrophy/hypertrophy cell signaling.

RESULTS

VIB training increased the maximal isometric unilateral leg extension force by 48·1%. ISO-K training improved the force by 24·8%. Both improvements were statistically significant (P⩿0·01). The more functional effectiveness of the VIB training in comparison with the ISO-K training was shown by the statistical significance changes only in VIB group in: rate of force development in time segment 0-50 ms (P<0·001), squat jump (P<0·05) and 30-m acceleration running test (P<0·05). VIB training induced a highly significant increase of mean diameter of fast fiber (+9%, P<0·001), but not of slow muscle fibers (-3%, not significant). No neural cell adhesion molecule-positive (N-CAM(+)) and embryonic myosin heavy chain-positive (MHC-emb(+)) myofibers were detected. VIB induced a significant twofold increase (P<0·05) of the skeletal muscle isoform insulin-like growth factor-1 (IGF-1) Ec mRNA. Atrogin-1 and muscle ring finger-1 (MuRF-1) did not change, but myostatin was strongly downregulated after VIB training (P<0·001). Peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) expression increased in post-training groups, but only in VIB reached statistical significance (+228%, P<0·05).

DISCUSSION

We demonstrated that both trainings are effective and do not induce muscle damage. Only VIB-trained group showed statistical significance increase of hypertrophy cell signaling pathways (IGF-1Ec and PGC-1α upregulation, and myostatin downregulation) leading to hypertrophy of fast twitch muscle fibers.

Current concepts of leg lengthening

Any lower limb discrepancy may be equalised by conservative means (insoles, prosthesis and orthosis). However, their long-term acceptance is low in regard to function, costs, expenditure and appearance. Timely epiphysiodesis is the best option in uniplanar deformities with adequate remaining growth and for patients whose predicted final body height is above the 50th percentile. However, many patients present late or with multi-planar deformities, which warrant more sophisticated operative approaches. The history of surgical bone lengthening comprises 100 exciting years of struggling, development and ongoing learning. The initial strategy of acute or rapid incremental distraction had lasted almost half a century until Ilizarov recognised the benefits of biological periosteum-preserving osteotomies and incremental lengthening at slow rates (1 mm/day) at a 4 × 0.25-mm daily rhythm, well appreciated as callotasis. In parallel, ring and wire constructs made complex three-dimensional axial, translational and rotational bone moulding possible. Taylor Spatial Frames-built on hexapod strut-linked platform technology as known from flight simulators-took limb correction to a more reliable, more precise and aesthetical level, all the more that the whole process became web-based. It represents state-of-the-art methodology and technology for complex, multi-plane deformities. Due to the significant risk of secondary malalignment, indications for lengthening by unilateral fixation have shrunken to moderate amounts of length disparity and uni- to bi-planar deformities in patients with still open physes. Mechanical or motorised, minimally invasively placed nails prevent muscle fixation and, therefore, ease rehabilitation, increase patient comfort and potentially shorten the overall time of sick leave and refrain from sports activities. Hence, they offer a valuable alternative for low-grade complexity situations. It remains to be proved if the significantly higher implant costs are compensated by lower treatment costs. Overall, limb lengthening, particularly in combination with multi-planar deformity correction, can still be an arduous endeavour. In any case, wise judgement of the patient's deformity, medical and biological situation, psychosocial environment, selection of the appropriate method and hardware, as well as meticulous operating technique by an experienced surgeon are the cornerstones of successful outcomes.

Cell-autonomous regulation of fast troponin T pre-mRNA alternative splicing in response to mechanical stretch

How mechanochemical signals induced by the amount of weight borne by the skeletal musculature are translated into modifications to muscle sarcomeres is poorly understood. Our laboratory recently demonstrated that, in response to experimentally induced increases in the weight load borne by a rat, alternative splicing of the fast skeletal muscle troponin T (Tnnt3) pre-mRNA in gastrocnemius was adjusted in a correlated fashion with the amount of added weight. (Schilder RJ, Kimball SR, Marden JH, Jefferson LS. J Exp Biol 214: 1523-1532, 2011). Thus muscle load is perceived quantitatively by the body, and mechanisms that sense it appear to control processes that generate muscle sarcomere composition plasticity, such as alternative pre-mRNA splicing. Here we demonstrate how mechanical stretch (see earlier comment) of C2C12 muscle cells in culture results in changes to Tnnt3 pre-mRNA alternative splicing that are qualitatively similar to those observed in response to added weight in rats. Moreover, inhibition of Akt signaling, but not that of ERK1/2, prevents the stretch-induced effect on Tnnt3 pre-mRNA alternative splicing. These findings suggest that effects of muscle load on Tnnt3 pre-mRNA alternative splicing are controlled by a cell-autonomous mechanism, rather than systemically. They also indicate that, in addition to its regulatory role in protein synthesis and muscle mass plasticity, Akt signaling may regulate muscle sarcomere composition by modulating alternative splicing events in response to load. Manipulation of Tnnt3 pre-mRNA alternative splicing by mechanical stretch of cells in culture provides a model to investigate the biology of weight sensing by skeletal muscles and facilitates identification of mechanisms through which skeletal muscles match their performance and experienced load.

In ovo temperature manipulation differentially influences limb musculoskeletal development in two lines of chick embryos selected for divergent growth rates

Selective breeding has led to diverging phenotypic evolution in layer and broiler chickens through genomic and epigenetic modifications. Here we show that in ovo environmental manipulation differentially influences embryonic limb muscle phenotype in these two breeds. We demonstrate that raising incubation temperature from 37.5 to 38.5°C between embryonic days (ED) 4 and 7 increased motility and body mass in both layer and broiler embryos. In layers, this was accompanied by gastrocnemius muscle hypertrophy, increased fibre and nuclei numbers and a higher nuclei to fibre ratio (ED18), preceded by increased hindlimb Myf5 (ED5–8), Pax7 (ED5–10), BMP4 (ED6–9) and IGF-I (ED9–10, ED18) mRNAs. In broilers, the same temperature treatment led to reduced gastrocnemius cross-sectional area with fewer fibres and nuclei and an unchanged fibre to nuclei ratio (ED18). This was preceded by a delay in the peak of hindlimb Myf5 expression, increased Pax7 (ED5, ED7–10) and BMP4 (ED6–8) but reduced IGF-I (ED8–10) mRNAs. Rather than promoting myogenesis as in layer embryos, the temperature treatment promoted gastrocnemius intramuscular fat deposition in broilers (ED18) preceded by increased hindlimb PPARγ mRNA (ED7–10). The treatment increased tibia/tarsus bone length as well as femur cross-sectional area in both breeds, but femur length and bone to cartilage ratio in the femur and tibia/tarsus were only increased in treated layers (ED18). We conclude that in ovo temperature manipulation differentially affected the molecular regulation of hindlimb myogenic, adipogenic and growth factor expression in broiler and layer embryos, leading to differential changes in muscle phenotype. The underlying interactive mechanisms between genes and the environment need further investigation.

Response of masticatory muscles to passive stretch stimulus - from perspectives of functional appliances

Objective

The aims of this study were to examine whether a passive stretch stimulus by means of a functional appliance induces changes in the fiber composition of masticatory muscles and whether these changes are similar to the changes in stretched limb muscle fibers by using RT-PCR, western blot, and immunohistochemical assays.

Methods

Five male New Zealand White rabbits were fitted with a prefabricated inclined plane on the maxillary central incisors to force the mandible forward (- 2 mm) and downward (- 4 mm). Further, 1 hind limb was extended and constrained with a cast so that the extensor digitorum longus (EDL) was stretched when the animal used the limb. The animals were sacrificed after 1 week and the masseter, lateral pterygoid, and EDL were processed and compared with those from control animals (n = 3).

Results

The stretched EDL had a significantly higher percentage of slow fibers, whereas the stretched masticatory muscles did not show changes in the composition of the major contractile proteins after 7 days.

Conclusions

The transition of fiber phenotypes in response to a stretch stimulus may take longer in the masticatory muscles than in the limb muscles.

Mechanical loading of stem cells for improvement of transplantation outcome in a model of acute myocardial infarction: the role of loading history

Stem cell therapy for tissue repair is a rapidly evolving field and the factors that dictate the physiological responsiveness of stem cells remain under intense investigation. In this study we hypothesized that the mechanical loading history of muscle-derived stem cells (MDSCs) would significantly impact MDSC survival, host tissue angiogenesis, and myocardial function after MDSC transplantation into acutely infarcted myocardium. Mice with acute myocardial infarction by permanent left coronary artery ligation were injected with either nonstimulated (NS) or mechanically stimulated (MS) MDSCs. Mechanical stimulation consisted of stretching the cells with equibiaxial stretch with a magnitude of 10% and frequency of 0.5 Hz. MS cell-transplanted hearts showed improved cardiac contractility, increased numbers of host CD31+ cells, and decreased fibrosis, in the peri-infarct region, compared to the hearts treated with NS MDSCs. MS MDSCs displayed higher vascular endothelial growth factor expression than NS cells in vitro. These findings highlight an important role for cyclic mechanical loading preconditioning of donor MDSCs in optimizing MDSC transplantation for myocardial repair.

Unloading stress disturbs muscle regeneration through perturbed recruitment and function of macrophages

Skeletal muscle is one of the most sensitive tissues to mechanical loading, and unloading inhibits the regeneration potential of skeletal muscle after injury. This study was designed to elucidate the specific effects of unloading stress on the function of immunocytes during muscle regeneration after injury. We examined immunocyte infiltration and muscle regeneration in cardiotoxin (CTX)-injected soleus muscles of tail-suspended (TS) mice. In CTX-injected TS mice, the cross-sectional area of regenerating myofibers was smaller than that of weight-bearing (WB) mice, indicating that unloading delays muscle regeneration following CTX-induced skeletal muscle damage. Delayed infiltration of macrophages into the injured skeletal muscle was observed in CTX-injected TS mice. Neutrophils and macrophages in CTX-injected TS muscle were presented over a longer period at the injury sites compared with those in CTX-injected WB muscle. Disturbance of activation and differentiation of satellite cells was also observed in CTX-injected TS mice. Further analysis showed that the macrophages in soleus muscles were mainly Ly-6C-positive proinflammatory macrophages, with high expression of tumor necrosis factor-α and interleukin-1β, indicating that unloading causes preferential accumulation and persistence of proinflammatory macrophages in the injured muscle. The phagocytic and myotube formation properties of macrophages from CTX-injected TS skeletal muscle were suppressed compared with those from CTX-injected WB skeletal muscle. We concluded that the disturbed muscle regeneration under unloading is due to impaired macrophage function, inhibition of satellite cell activation, and their cooperation.

Long-term voluntary exercise, representing habitual exercise, lowers visceral fat and alters plasma amino acid levels in mice

OBJECTIVES

To determine the impact of long-term voluntary exercise, representing habitual exercise for the prevention of lifestyle-related diseases, on glucose, lipid, and amino acid metabolism in mice.

METHODS

Twenty-four mice aged 6 weeks were divided into three groups. Two groups (16 mice) were housed individually in either cages equipped with a running wheel (8 mice, exercising, Ex-mice) or without (8 mice, sedentary, Se-mice) for 24 weeks. The remaining group (8 mice) was sacrificed at 6 weeks of age. Biomarkers related to glucose, lipid, and amino acid metabolism were examined.

RESULTS

Ex-mice ran voluntarily, predominantly in the dark. The distance per day peaked at 4 weeks and then decreased until 12 weeks to around the level seen at the beginning of the experimental period, and was maintained at 4.9 ± 0.2 km/day from 12 to 24 weeks. Ex-mice showed a similar adrenal weight and vitamin C content to Se-mice but had a significantly lower body weight and higher food intake. Ex-mice also showed a higher skeletal muscle weight, a lower white adipose tissue and liver weight, associated with lower plasma leptin and insulin-like growth factor-1 levels, and a lower hepatic triglyceride content. Analysis of plasma amino acids showed that Ex-mice had significantly higher phenylalanine, tyrosine, and glutamine levels, resulting in a significantly lower Fischer's ratio.

CONCLUSIONS

We present an animal model of long-term voluntary exercise under low stress. Findings related to the effects of long-term voluntary exercise on lipid, and amino acid metabolism in our mouse model indicate that such an exercise regimen may affect pathophysiological states related to appetite and behavior.

Static mechanical stretching accelerates lipid production in 3T3-L1 adipocytes by activating the MEK signaling pathway

Understanding mechanotransduction in adipocytes is important for research of obesity and related diseases. We cultured 3T3-L1 preadipocytes on elastic substrata and applied static tensile strains of 12% to the substrata while inducing differentiation. Using an image processing method, we monitored lipid production for a period of 3-4 wk. The ratio of %-lipid area per field of view (FOV) in the stretched over nonstretched cultures was significantly greater than unity (P < 0.05), reaching ∼1.8 on average starting from experimental day ∼10. The superior coverage of the FOV by lipids in the stretched cultures was due to significantly greater sizes of lipid droplets (LDs) with respect to nonstretched cultures, starting from experimental day ∼10 (P < 0.05), and due to significantly more LDs per cell between days ∼10 and ∼17 (P < 0.05). The statically stretched cells also differentiated significantly faster than the nonstretched cells within the first ∼10 days (P < 0.05). Adding peroxisome proliferator-activated receptor-γ (PPARγ) antagonist did not change these trends, as the %-lipid area per FOV in the stretched cultures that received this treatment was still significantly greater than in the nonstretched cultures without the PPARγ antagonist (14.44 ± 1.96% vs. 10.21 ± 3%; P < 0.05). Hence, the accelerated adipogenesis in the stretched cultures was not mediated through PPARγ. Nonetheless, inhibiting the MEK/MAPK signaling pathway reduced the extent of adipogenesis in the stretched cultures (13.53 ± 5.63%), bringing it to the baseline level of the nonstretched cultures without the MEK inhibitor (10.21 ± 3.07%). Our results hence demonstrate that differentiation of adipocytes can be enhanced by sustained stretching, which activates the MEK signaling pathway.

Absence of heat shock transcription factor 1 retards the regrowth of atrophied soleus muscle in mice

Effects of heat shock transcription factor 1 (HSF1) gene on the regrowth of atrophied mouse soleus muscles were studied. Both HSF1-null and wild-type mice were subjected to continuous hindlimb suspension for 2 wk followed by 4 wk of ambulation recovery. There was no difference in the magnitude of suspension-related decrease of muscle weight, protein content, and the cross-sectional area of muscle fibers between both types of mice. However, the regrowth of atrophied soleus muscle in HSF1-null mice was slower compared with that in wild-type mice. Lower baseline expression level of HSP25, HSC70, and HSP72 were noted in soleus muscle of HSF1-null mice. Unloading-associated downregulation and reloading-associated upregulation of HSP25 and HSP72 mRNA were observed not only in wild-type mice but also in HSF1-null mice. Reloading-associated upregulation of HSP72 and HSP25 during the regrowth of atrophied muscle was observed in wild-type mice. Minor and delayed upregulation of HSP72 at mRNA and protein levels was also seen in HSF1-null mice. Significant upregulations of HSF2 and HSF4 were observed immediately after the suspension in HSF1-null mice, but not in wild-type mice. Therefore, HSP72 expression in soleus muscle might be regulated by the posttranscriptional level, but not by the stress response. Evidence from this study suggested that the upregulation of HSPs induced by HSF1-associated stress response might play, in part, important roles in the mechanical loading (stress)-associated regrowth of skeletal muscle.

Blood flow restriction: the metabolite/volume threshold theory

Traditionally it has been thought that muscle hypertrophy occurs primarily from an overload stimulus produced by progressively increasing an external load using at least 70% of one's concentric one repetition maximum (1RM). Blood flow restricted exercise has been demonstrated to result in numerous positive training adaptions, specifically muscle hypertrophy and strength at intensities much lower than this recommendation. The mechanisms behind these adaptions are currently unknown but a commonly cited concept is that acute elevations of systemic hormones, specifically growth hormone (GH), play a large role with resistance training induced muscle hypertrophy, possibly through stimulating muscle protein synthesis (MPS). We hypothesize that the alterations in the intramuscular environment which results in the rapid recruitment of FT fibers, is the large driving force behind the skeletal muscle hypertrophy seen with blood flow restriction, whereas the external load and systemic endogenous hormone elevations may not be as important as once thought. It is further hypothesized that although skeletal muscle hypertrophy can be achieved at low intensities without blood flow restriction when taken to muscular failure, the overall volume of work required is much greater than that needed with blood flow restriction.

Transcriptional mechanisms regulating skeletal muscle differentiation, growth and homeostasis

Skeletal muscle is the dominant organ system in locomotion and energy metabolism. Postnatal muscle grows and adapts largely by remodelling pre-existing fibres, whereas embryonic muscle grows by the proliferation of myogenic cells. Recently, the genetic hierarchies of the myogenic transcription factors that control vertebrate muscle development - by myoblast proliferation, migration, fusion and functional adaptation into fast-twitch and slow-twitch fibres - have become clearer. The transcriptional mechanisms controlling postnatal hypertrophic growth, remodelling and functional differentiation redeploy myogenic factors in concert with serum response factor (SRF), JUNB and forkhead box protein O3A (FOXO3A). It has also emerged that there is extensive post-transcriptional regulation by microRNAs in development and postnatal remodelling.

Early effects of muscle atrophy on shoulder joint development in infants with unilateral birth brachial plexus injury

AIM

Shoulder deformities in children with a birth brachial plexus injury (BBPI) are caused by muscle imbalances; however, the underlying mechanisms are unclear. The aim of this study was to assess the early interactions between shoulder muscles and shoulder joint development.

METHOD

In a retrospective magnetic resonance imaging (MRI) study of 36 infants (21 males, 15 females) younger than 12 months (mean 4.8 mo) with unilateral BBPI, volumes and thicknesses of standardized segments of the infraspinatus, subscapularis, and deltoid muscles were measured in both shoulders and expressed as ratios of pathological/unaffected side. The relation between muscle ratios and humeral head subluxation, passive external rotation, glenoid version, and deformity was analysed.

RESULTS

Compared with the unaffected side, the muscles of the affected side were of significantly smaller volume and thickness. The subscapularis was the most severely affected muscle, its volume being only 64% (SD 21%) and its thickness only 79% (SD 23%) of the corresponding values on the unaffected side (p < 0.001). Severe subluxation was predicted by the combination of low infraspinatus and subscapularis volume ratios (r(2) = 0.223; p = 0.014), but not by muscle thickness ratios. Subluxation was related to passive external rotation (p < 0.05), glenoid version (p < 0.01), and deformity (p < 0.01).

INTERPRETATION

In infants with BBPI, muscle size is decreased during in the first months of life by both atrophy and, possibly, by a reduction in the number of sarcomeres in series. These effects are strongly related to shoulder joint subluxation.